This rule applies to jackpot games only.
When a player is leaving a game and begins to rack chips( because they are leaving a game), they may play ONE hand before they will be dealt out and must leave the table.
If they begin racking prior to the beginning of a hand they may play the next hand, if they begin racking during a hand in progress( in which they are still involved) THAT will be their last hand.
Many players are under the impression that once they begin racking their chips, they are entitled to play an additional 3 hands before they must leave they game or un-rack the chips. This has never actually been the rule, the actual rule has just never been consistently enforced.