Tetrion

rotation explained for O piece:

case k_TetStandardPieces_PieceID_O:
	piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
	Create4Minos(piece,pIsSticky,0,0,1,0,0,1,1,1);

this part defines the minos
(x0,y0) , (x1,y1) , (x2,y2) , (x3,y3)
( 0, 0) , ( 1, 0) , ( 0, 1) , ( 1, 1)

the bigger y, the more the mino is located towards the top
so the O piece is per default (state=0) located in the top right corner of the origin

pieces are always turned around the origin, in clockwise direction it's:
xNew = y
yNew = -x
( 0, 0) , ( 0,-1) , ( 1, 0) , ( 1,-1)
so after clockwise rotation (state=1), O piece is located in bottom right corner of the origin

if you rotate again in clockwise rotation, then you get
(0 , 0) , (-1, 0) , ( 0,-1) , (-1,-1)
so after rotating twice (state=2), O piece is located in bottom left corner of the origin

and a third time:
(0 , 0) , ( 0, 1) , (-1, 0) , (-1, 1)
so after rotating three times (state=3), O piece is located in top left corner of the origin
it's the same as if you would have rotated the original coordinates in counterclockwise direction:
xNew = -y
yNew = x

case k_TetStandardPieces_PieceID_O:
	rotationDef.AddRotationPoint(0,0,0,-1,-1,-1,-1,0);

this part defines the kicks / rotation offsets
(x0,y0) , (x1,y1) , (x2,y2) , (x3,y3)
( 0, 0) , ( 0,-1) , (-1,-1) , (-1, 0)
(x0,y0) represent the offsets in state 0 (initial orientation)
(x1,y1) represent the offsets in state 1 (orientation after clockwise rotation)
(x2,y2) represent the offsets in state 2 (orientation after 180 degree rotation)
(x3,y3) represent the offsets in state 3 (orientation after counterclockwise rotation)

when rotating from state 0 to 1, the game will kick the O piece's center by
(x0 - x1, y0 - y1) = (0, 1)
that means the piece is kicked one row upwards
this basically negates the change of the minos' positions (top right -> bottom right)

when rotating from state 1 to 2, the game will kick the O piece's center by
(x1 - x2, y1 - y2) = (1, 0)
that means the piece is kicked one column to the right
this basically negates the change of the minos' positions (bottom right -> bottom left)

when rotating from state 2 to 3, the game will kick the O piece's center by
(x2 - x3, y2 - y3) = (0,-1)
that means the piece is kicked one row downwards
this basically negates the change of the minos' positions (bottom left -> top left)

so the O piece always stays in its place despite of

Other pieces have more than 1 rotation point. For example the T,L,J,S,Z pieces' first rotation point only consists of 0s, which means the center of those pieces is not changed during rotation. However, if the base rotation fails, the game will try a non-0 rotation point which represent the first kick for each rotation. If the second rotation point also fails, then the game will try the third rotation point (which represents the second kick for each rotation), and so on.

code for the Flash games that were part of Tetris Online Poland. Games at Tetris Friends should have the same code but it is obfuscated:

public class TetStandardPieces
   {

      public static const k_TetStandardPieces_PieceID_3I:int = 7;
      public static const k_TetStandardPieces_PieceID_3J:int = 8;
      public static const k_TetStandardPieces_PieceID_3L:int = 9;
      public static const k_TetStandardPieces_PieceID_I:int = 4;
      public static const k_TetStandardPieces_PieceID_J:int = 5;
      public static const k_TetStandardPieces_PieceID_L:int = 1;
      public static const k_TetStandardPieces_PieceID_O:int = 2;
      public static const k_TetStandardPieces_PieceID_S:int = 3;
      public static const k_TetStandardPieces_PieceID_T:int = 6;
      public static const k_TetStandardPieces_PieceID_2I:int = 10;
      public static const k_TetStandardPieces_PieceID_Z:int = 0;
      public static const k_TetStandardPiecesNumPieceIDs:int = 7;

      private static function Create2Minos(p_Piece:TetPiece, pIsSticky:Boolean, p_Mino0_X:int, p_Mino0_Y:int, p_Mino1_X:int, p_Mino1_Y:int) : void
      {
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino0_X,p_Mino0_Y);
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino1_X,p_Mino1_Y);
      }

      private static function Create4Minos(p_Piece:TetPiece, pIsSticky:Boolean, p_Mino0_X:int, p_Mino0_Y:int, p_Mino1_X:int, p_Mino1_Y:int, p_Mino2_X:int, p_Mino2_Y:int, p_Mino3_X:int, p_Mino3_Y:int) : void
      {
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino0_X,p_Mino0_Y);
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino1_X,p_Mino1_Y);
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino2_X,p_Mino2_Y);
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino3_X,p_Mino3_Y);
      }

      private static function Create3Minos(p_Piece:TetPiece, pIsSticky:Boolean, p_Mino0_X:int, p_Mino0_Y:int, p_Mino1_X:int, p_Mino1_Y:int, p_Mino2_X:int, p_Mino2_Y:int) : void
      {
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino0_X,p_Mino0_Y);
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino1_X,p_Mino1_Y);
         p_Piece.AddMino(new TetMino(p_Piece.GetPieceTypeID(),pIsSticky,null),p_Mino2_X,p_Mino2_Y);
      }

      private static function CreateNewRotationDef(p_PieceID:int) : TetSuperRotationDef
      {
         var rotationDef:TetSuperRotationDef = new TetSuperRotationDef(null);
         switch(p_PieceID)
         {
            case k_TetStandardPieces_PieceID_S:
            case k_TetStandardPieces_PieceID_Z:
            case k_TetStandardPieces_PieceID_J:
            case k_TetStandardPieces_PieceID_L:
            case k_TetStandardPieces_PieceID_T:
               rotationDef.AddRotationPoint(0,0,0,0,0,0,0,0);
               rotationDef.AddRotationPoint(0,0,1,0,0,0,-1,0);
               rotationDef.AddRotationPoint(0,0,1,-1,0,0,-1,-1);
               rotationDef.AddRotationPoint(0,0,0,2,0,0,0,2);
               rotationDef.AddRotationPoint(0,0,1,2,0,0,-1,2);
               break;
            case k_TetStandardPieces_PieceID_O:
               rotationDef.AddRotationPoint(0,0,0,-1,-1,-1,-1,0);
               break;
            case k_TetStandardPieces_PieceID_I:
               rotationDef.AddRotationPoint(0,0,-1,0,-1,1,0,1);
               rotationDef.AddRotationPoint(-1,0,0,0,1,1,0,1);
               rotationDef.AddRotationPoint(2,0,0,0,-2,1,0,1);
               rotationDef.AddRotationPoint(-1,0,0,1,1,0,0,-1);
               rotationDef.AddRotationPoint(2,0,0,-2,-2,0,0,2);
               break;
            case k_TetStandardPieces_PieceID_3L:
               rotationDef.AddRotationPoint(0,0,0,-1,-1,-1,-1,0);
               rotationDef.AddRotationPoint(0,0,1,-1,0,-2,-1,-1);
               rotationDef.AddRotationPoint(1,-1,0,-2,-1,-1,0,0);
               break;
            case k_TetStandardPieces_PieceID_3J:
               rotationDef.AddRotationPoint(0,0,1,0,1,-1,0,-1);
               rotationDef.AddRotationPoint(0,0,1,-1,0,-2,-1,-1);
               rotationDef.AddRotationPoint(-1,-1,0,0,1,-1,0,-2);
               break;
            case k_TetStandardPieces_PieceID_3I:
               rotationDef.AddRotationPoint(0,0,0,0,0,0,0,0);
               rotationDef.AddRotationPoint(1,0,0,0,-1,0,0,0);
               rotationDef.AddRotationPoint(-1,0,0,0,1,0,0,0);
               rotationDef.AddRotationPoint(0,0,0,1,0,-1,0,0);
               rotationDef.AddRotationPoint(0,0,0,-1,0,1,0,0);
               break;
            case k_TetStandardPieces_PieceID_2I:
               rotationDef.AddRotationPoint(0,0,-1,0,-1,1,0,1);
               rotationDef.AddRotationPoint(0,0,-1,-1,-2,0,-1,1);
               rotationDef.AddRotationPoint(0,0,0,0,0,0,0,0);
               rotationDef.AddRotationPoint(0,0,0,-1,-1,0,-1,-1);
         }
         return rotationDef;
      }

      public static function CreateNewPiece(p_PieceID:int, pIsSticky:Boolean = false) : TetPiece
      {
         var piece:TetPiece = null;
         switch(p_PieceID)
         {
            case k_TetStandardPieces_PieceID_S:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,0,1,-1,0,1,1);
               break;
            case k_TetStandardPieces_PieceID_Z:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,0,1,1,0,-1,1);
               break;
            case k_TetStandardPieces_PieceID_J:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,-1,0,1,0,-1,1);
               break;
            case k_TetStandardPieces_PieceID_L:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,1,0,-1,0,1,1);
               break;
            case k_TetStandardPieces_PieceID_T:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,0,1,-1,0,1,0);
               break;
            case k_TetStandardPieces_PieceID_O:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,1,0,0,1,1,1);
               break;
            case k_TetStandardPieces_PieceID_I:
               piece = new TetPiece(p_PieceID,4,CreateNewRotationDef(p_PieceID),null);
               Create4Minos(piece,pIsSticky,0,0,1,0,2,0,-1,0);
               break;
            case k_TetStandardPieces_PieceID_3I:
               piece = new TetPiece(p_PieceID,3,CreateNewRotationDef(p_PieceID),null);
               Create3Minos(piece,pIsSticky,0,0,1,0,-1,0);
               break;
            case k_TetStandardPieces_PieceID_3J:
               piece = new TetPiece(p_PieceID,3,CreateNewRotationDef(p_PieceID),null);
               Create3Minos(piece,pIsSticky,0,0,0,1,-1,0);
               break;
            case k_TetStandardPieces_PieceID_3L:
               piece = new TetPiece(p_PieceID,3,CreateNewRotationDef(p_PieceID),null);
               Create3Minos(piece,pIsSticky,0,0,0,1,1,0);
               break;
            case k_TetStandardPieces_PieceID_2I:
               piece = new TetPiece(p_PieceID,2,CreateNewRotationDef(p_PieceID),null);
               Create2Minos(piece,pIsSticky,0,0,1,0);
         }
         return piece;
      }
   }

   public class TetSuperRotationDef extends TetObject
   {

      private static const k_TetMaxNumRotationPoints:int = 5;
      private var m_RotationPoints:Array;
      private var m_NumRotationPoints:int;

      public function TetSuperRotationDef(p_RotationDefToCopy:TetSuperRotationDef)
      {
         super();
         m_RotationPoints = new Array(TetMatrix2D.k_TetNumFacings * k_TetMaxNumRotationPoints);
         var point:int = 0;
         var facing:int = 0;
         var index:int = 0;
         if(p_RotationDefToCopy == null)
         {
            m_NumRotationPoints = 0;
            for(point = 0; point < k_TetMaxNumRotationPoints; point++)
            {
               for(facing = 0; facing < TetMatrix2D.k_TetNumFacings; facing++)
               {
                  m_RotationPoints[index] = null;
                  index++;
               }
            }
         }
         else
         {
            m_NumRotationPoints = p_RotationDefToCopy.GetNumRotationPoints();
            for(point = 0; point < m_NumRotationPoints; point++)
            {
               for(facing = 0; facing < TetMatrix2D.k_TetNumFacings; facing++)
               {
                  m_RotationPoints[index] = new TetVector2D(0,0,p_RotationDefToCopy.m_RotationPoints[index]);
                  index++;
               }
            }
         }
      }

      public function GetNumRotationPoints() : int
      {
         return m_NumRotationPoints;
      }

      public function AddRotationPoint(p_FacingN_X:int, p_FacingN_Y:int, p_FacingE_X:int, p_FacingE_Y:int, p_FacingS_X:int, p_FacingS_Y:int, p_FacingW_X:int, p_FacingW_Y:int) : void
      {
         if(m_NumRotationPoints >= k_TetMaxNumRotationPoints)
         {
            return;
         }
         var facingIndex:int = m_NumRotationPoints * TetMatrix2D.k_TetNumFacings;
         m_RotationPoints[facingIndex] = null;
         m_RotationPoints[facingIndex] = new TetVector2D(p_FacingN_X,p_FacingN_Y,null);
         facingIndex++;
         m_RotationPoints[facingIndex] = null;
         m_RotationPoints[facingIndex] = new TetVector2D(p_FacingE_X,p_FacingE_Y,null);
         facingIndex++;
         m_RotationPoints[facingIndex] = null;
         m_RotationPoints[facingIndex] = new TetVector2D(p_FacingS_X,p_FacingS_Y,null);
         facingIndex++;
         m_RotationPoints[facingIndex] = null;
         m_RotationPoints[facingIndex] = new TetVector2D(p_FacingW_X,p_FacingW_Y,null);
         m_NumRotationPoints++;
      }
	}