The effect is that a small group of picture cards are displayed on the iPhone. You are asked to think of one, but not indicate which one you have chosen, and no tapping on the card on the iPhone screen so people don't think that is the secret. There is some nice animation as the cards flip over and one is removed. All the cards are face down. You are asked to reveal your card, and all the cards are turned over but one. Your card is no longer in the group.
It's a nice trick, and an old one. It's generally knows as the Princess Card trick, and after years of being a mainstay for magicians it made it to the web and was widely seen in a number of variations.
This is a nice version of the effect, and the animation is smooth and attractive. My only criticisms are that the magician gets no credit for the trick. it appears that the phone is doing all the heavy lifting. Also, the card left over, the face down one, can't be revealed. I think there are some ways to deal with that if the developer takes the trouble to work it out.
For people who have not seen the trick, it will be a stunner. It's a good little diversion to try in a bar or a party. If you think the secret is too widespread, this is not the trick for you. If the description seems fresh, give it a try on your friends or co-workers.