Another trick from the classic era of magic! An assistant steps into the box, and then the magician seems to break them into three pieces. Usually, their torso is shifted to the extreme left or right - their body appears to be completely disconnected.
If this were possible, magicians would ask for volunteers from the audience to do it. They don't, though - they need an exceptional contortionist who can just-about bend themselves within the box enough to make the illusion possible.