The buzzsaw trick is a lot like the 'cutting someone in half' trick, but taken to the next level. It's also a lot more dangerous than the 'cutting someone in half' trick because the buzzsaw is very real.
The secret to this is that the feet at the end of the box are dummy feet. All the person in the box has to do is get their real feet and legs out of the way before the buzzsaw slices through them. It can still go very wrong if their timing is out.