I can explain

First, try holding a coin in your left hand and throwing it up.
That's easy.
Now, try and do it but starting with the left hand palm-down (with the coin palmed) so you have to turn over the hand.
Easy.
Now, try to throw it up while making the least motion possible with the left hand. You'll have to turn it over about 90 degrees and make a flicking motion with your wrist. Of couse, this should be fast enough that it just looks like a flick.
Now, flick up an invisible coin and come down to slap it on the back of the left hand. As you're coming down, flick the coin in the left hand up. It should hit the right hand and be slapped down on the back of your hand.
Even if the speccy catches a glimpse of the coin, the throw-up is so hard to see that it just looks like the coin appeared

Kati