In this simple mind reading trick, Criss Angel asks a female spectator to put a ring under any of the three cups on the table and then switch the other two cups. He then finds out underneath which cup she put the ring, seemingly just by observing her body language.
While this is a pretty simple trick, it can serve as a great mind reading effect. With the right showmanship, you can convince your spectators that you actually read their minds (or body language) to find out underneath which cup they put the ring. However, this trick has nothing to do with mind reading or body language. The secret is simply that one of the cups is different! Not very different, of course, but it must have some little detail that will help you distinguish it from other two cups. The cup which is different is called “the key cup”.
Once you are aware of this difference, the rest is easy. After the ring has been hidden, you simply check whether your key cup has moved. The position of the key cup tells you where the ring is.
Criss explains all the possibilities in the next video: