The player hopes to mislead the opener by rapping pat out of turn.
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There's probably no penalty if she changes her mind, because general poker rules dictate that "acting out of turn" is not binding. Even if you can interpret this tell correctly, it seldom does you much good in high-hand-wins draw poker. True, the opponent is trying to mislead you by pretending to rap pat, but her motive is fairly hazy. She wants to fool you, but in her heart she doesn't quite know why. You can be fairly certain that she'll draw cards, but what can you do about it? In lowball, the tell is more beneficial.
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If an opponent is pretending to rap pat behind you and your jacks-or-better-to-open hand is Ace of Diamonds, Ace of Clubs, 8 of Clubs, 7 of Clubs, 2 of Clubs, you should definitely draw three to the aces and not be tricked into splitting openers and trying for the flush. If the game is lowball, there's a much greater chance that you can earn a profit here. If you have a hand such as 9, 7, 6, 4, 2 and were tempted to draw one, you should now rap pat, knowing that your lowball opponent will be drawing cards.
