The question is what does he do at the end if Alyson Hannigan picks the card that he saw himself tear up.
Alyson couldn't have picked the card that was torn up, because she selected a card from the deck after the card was destroyed; https://youtu.be/5_KcQt0z-eE?t=332 is around when she's given cards to select from.
On the other hand, Penn was asked to name a card which was then shown, an alternate card destroyed, and the named card shown again. If a further name a card was attempted on the 51-card deck, you'd have the chance of naming the destroyed card.
> It certainly isn't random.
It's random in the context of the trick, it's not like he specifically chose that card because he knew he could avoid it later
He's very good at what he does, and it's hard to spot in real time. Amazing card scanning, too.
(start around https://www.youtube.com/watch?v=5_KcQt0z-eE&t=229s )