I think there's a string running from his mouth to the card and then from the card to some point off to the left. I think that for four reasons:
First, the motion of his hands wouldn't interfere with such a setup.
Second, the card wobbles about an axis that suggests such a setup.
Third, he never opens his mouth.
Fourth, the card descends only when he leans forward.
Right, just a good trick, and I doubt that something "truly" levitating would so easily wobble. And the axis of wobble is such that, like said, would not cause his hand movements to interfere with the "invisible" string.
Still, good trick.
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