Serious amateur magician here (magician member of the Magic Castle). Note that the trick done for the monkeys is not done the way that a real magician would do the trick to fool a human. The French Drop actually can have surprisingly deep subtleties. To fool a human, you would hold the coin between your thumb and forefinger so that there is a visible gap between the bottom of the coin and the palm of the hand holding it into which it will be dropped. Then the hand that grabs the coin is moved into place in a smooth continuous motion, covering this gap very briefly, during which time the coin is dropped. The key is that the gap is visible during most of this motion, after which the grasping hand pauses briefly before closing the thumb and forefingers to "grasp" the (now missing) coin. The key to making it convincing is that the trick actually happens before the dramatic pause before grasping the coin, but the pause focuses the spectator's attention on the fact that during the pause the gap is visible, making it "impossible" for the coin to be dropped without being seen. It's considered one of the most elementary of tricks, but it's actually quite challenging to do properly. In the hands of a skilled magician it can be very convincing even when you know how it's done.

Magic is an art with vastly more subtlety and attention to detail than it generally appreciated by muggles.