$ft = New-Object System.TimeSpan(10000)
$file.Lock([Microsoft.SharePoint.SPFile+SPLockType]::Exclusive, "test lock", $ft)
$file.UndoCheckOut()

$web = Get-SPWeb http://intranet.contoso.com
$list = $web.Lists["DocLib"]
$item = $list.GetItemById(2)
$file = $item.File
$userId = $file.LockedByUser.ID
$user = $web.AllUsers.GetByID($userId)
$impSite= New-Object Microsoft.SharePoint.SPSite($web.Url, $user.UserToken);
$impWeb = $impSite.OpenWeb();
$impList = $impWeb.Lists[$list.Title]
$impItem = $impList.GetItemById($item.ID)
$impFile = $impItem.File
$impFile.ReleaseLock($impFile.LockId)