Criss does not use some sort of magical power, as he himself has stated. This is how it's done, according to LearnStreetMagic.com:
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Anyways, here's the secret to Criss Angel’s trick: the lady was an amputee. Having a hard time believing that? Go google, “Rose Siggins.” The lady on the bottom is a contortionist (bent over, hands on legs). The amputee sits on top of her and the two are held together by straps, clips, magnets or something. Note how the lady is conveniently wearing a black dress that would disguise where the two people join. Of course, some post editing would have concealed anything that might have been flashed.
The amputee is obviously an amputee, as she does an amateur acting job, as well as running (well, arming) away for some reason, rather than seeking help to get re-attached. Plus, how is it that a normal person has that much upper body strength and that smooth movement while arming around with half a body for the first time. Case closed.