== Null Sprite ==
Routine at $01F14B is called, Y = #$FF from $01F15A: "LDY $160E,X"
Bunch of values are set according to Y.
Y is usually the sprite slot number of the sprite Yoshi has in his mouth.
X is the sprite slot of Yoshi.
Null sprite is id -1, or #$FF, so all tables are indexed way out of bounds.

PC         Instruction    Eff.Addr.    Intended Memory/Table            Result Memory/Table                            Value Stored

$01F203    STA $00E4,Y    $01E3        sprite X position low byte       stack                                          $00E4,X +/- #$10 (Yoshi X-pos low, depending on direction)
$01F210    STA $14E0,Y    $15DF        sprite X position high byte      sprite interact with objects table,#$03        $14E0,X - 0/1 (Yoshi X-pos high, depending on direction)
$01F215    STA $00D8,Y    $01D7        sprite Y position low byte       stack                                          $00D8,X (Yoshi Y-pos low)
$01F21B    STA $14D4,Y    $15D3        sprite Y position high byte      sprite about to be eaten table,#$03            $14D4,X (Yoshi Y-pos high)
$01F220    STA $00C2,Y    $01C1        misc sprite table                stack                                          #$00
$01F223    STA $15D0,Y    $16CF        sprite about to be eaten table   bounce sprite pos/size interaction table,#$02  #$00
$01F226    STA $1626,Y    $1725        sprite kill counter table        extended sprite X position low byte,#$06       #$00
$01F234    STA $14C8,Y    $15C7        sprite status table              sprite off-screen table,#$03                   #$09 if ducking, #$0A otherwise
$01F23B    STA $157C,Y    $167B        sprite horiz direction table     4th tweaker byte table,#$01                    $157C,X (Yoshi direction)
$01F246    STA $00B6,Y    $01B5        sprite X speed table             stack                                          +/- #$10 if ducking, +/- #$30 other wise (depending on direction)
$01F24B    STA $00AA,Y    $01A9        sprite Y speed table             stack                                          #$00

== Brown Platform Spit ==
Normally on execution of routine ($01F14B), Stack pointer is $01EB.
Stack is small enough that values that are written into the stack as a result of table index issue don't effect important values. ($01E3 is deepest errant address)
However, when routine is called on brown platform, stack frame is larger due to having to adjust positions with janky brown swinging platform.
Stack pointer is $01E2 upon calling $01F14B.
In this stack frame (#$01 | $01E4 | $01E3) is address routine will return to + 1. (uses RTS)
Therefore you can jump to different parts of code between $01EE01 and $01EF00 depending on Yoshi's X-pos low byte being between #$00 and #$FF upon spitting.

== Powerup Incrementation ==
With Yoshi X-pos low byte of #$C6 and facing right, we return execution to $01EED7.
This is a desynced PC value, which corresponds to JSR $9D68. We jump to $019D68.
This is also a desynced PC value, which corresponds to INC $15,X.
Remember X is Yoshi's sprite slot, so if Yoshi's slot is #$04, we incrment $0019, which is Mario's current powerup.
Like credits warp, execution syncs back up eventually after running through a sprite graphics routine.
This is why you see a dark Mario head under Yoshi for one frame when you sucessfully pull off PI.

Yoshi Slot   Eff.Addr.    Result

00           $0015        Right direction on d-pad is held for one frame
01           $0016        Right direction on d-pad is pressed
02           $0017        nothing
03           $0018        nothing
04           $0019        Mario's powerup increments. small -> big -> cape -> fire -> unused
05           $001A        layer 1 X-pos shifts by #$0001 for one frame
06           $001B        layer 1 X-pos shifts by #$0100 for one frame
07           $001C        layer 1 Y-pos shifts by #$0001 for one frame
08           $001D        layer 1 Y-pos shifts by #$0100 for one frame
09           $001E        layer 2 X-pos shifts by #$0001 for one frame
0A           $001F        layer 2 X-pos shifts by #$0100 for one frame
0B           $0020        layer 2 Y-pos shifts by #$0001 for one frame