Talk a lot more! All along the trick you were just going "Okay so this is the card, umm, ahh, so, umm, check the box!"
Look at d+M performing, he's talking about what's he's doing, and that's important, because YOU know what you're going to do, the audience don't. So if you go that fast, they can't catch up, the look in that girl's eyes was like "okay, what's he doing? Oh, he's ripping the card, but why?" and by then you already told her to check out the box.
Say something! It doesn't even matter if it makes sense or not, because no one will remember your specific words! d+M says in the clip "Look, hold on to that piece (emphasizing the fact that there are two pieces and the second one is in his hands), now, there's a moment in time where my piece vanishes... Did you see that moment in time?" See, that doesn't even make any sense! It doesn't matter, just say something, tell the audience it's going to dissapear and reappear inside the box, and most importantly - let them take and open the box themselves! And emphasize that, say "I'm not going to go near the box, just check it yourselves."
And lastly, don't get the card signed, it's a bit stupid... It works great in tricks like Believe where the signature is also ripped, but not here. If you want them not to think you've switched the torn corner, just emphasize on the fact that they should check really close that the corner really matches and that it's really the same card. It's all in your words. Talk more and emphasize the important things with your words.
Good luck!
