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4:39 you can see that she is going forward, but something else is holding up the sheet from behind. That something gets pulled away along with the sheet. That should give you a clue as to how it's done. He covers up a trap door in front of the chair when he pulls the sheet up the first time. She opens the trap door while it's pulled over the first time, but comes back into position to do the whole "my head is still here" thing. Then she goes through the trap door for the final part. Not his best illusion.
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