These are surprisingly strong magnets that are thin enough to hide in a playing card without providing any visual clue that they've been added. How? Why? I HIGHLY recommend Sin Lim's 52 Shades of Red which not only shows you how these might be useful but a whole lot more -- a magic education in the guise of a magic routine.
If you were blown away by Shades of Red on Penn and Teller's Fool Us, you'll be even more blown away by the fact that you can actually learn to perform it. But, better still, the remarkable video that accompanies it will teach magicians at every level how to improve their magic. (A set of these magnets, by the way, is included in 52SOR. But, you'll soon be thinking of other uses.