Could be a whole in the glass (which he's being careful to conceal with his other hand). Maybe the glass slides around (moving the hole into and out of the trick area). He does move his left arm into that area in a strage way.

Here's my guess: He has a fake coin which has two parts - one above the glass and one below - held together on either side of the glass by magnets. The fake coin is already set up under his left hand. He then switches it for the real coin by passing his left hand over the real coin (which he discards). From this point on the trick is basically done. He then uses his magnetic ring (right hand) to lift off the top part of the fake coin (the one above the glass), allowing the bottom half to fall onto his palm (the one under the glass) and - voila!