The glass table top has a hidden hole in it. When he picks up the salt shaker and then asks the girl to hold it under the table, watch his left hand - he puts it right at the edge of the glass, but the table continues for a foot or so towards the camera - the rest of the glass, with the hole in it, is in there, hidden inside the table. (It's one big piece of glass, bigger than the glass you can actually see). He then drags the GLASS from his left to his right, using the edge of his hand, with the part with the hole in it being hidden under his wrist as he does this (notice his sleeves are now down to his wrists). Then he covers the hole with his left hand, and pulls the salt shaker through the glass, through the hole. Watch what he does when he pulls the salt shaker out - he pulls his lift hand and ARM across to his left, which allows the glass to slide back (presumably under a spring) - the hole sliding back under the table, covered by his forearm - then he drops the salt shaker onto the glass top. Why would he perform that odd manoeuvre with his left arm otherwise?