This is the "trick" where you bend back the thumb on your left hand and bend foward (like making a fist) the thumb on your right. You put the two thumbs together and cover the seam between them with the first two fingers of your right hand so the thumb looks whole. Then you hold your hands up, and pull them apart and back together again quickly.
Depending on the age and sophistication of your audience, this can be shocking, delightful or simply lame.