You must not be a fan of
xkcd...
But seriously, the first bit of the performance is pretty easy to figure out. There's a guy bent over with his upper torso in the table. Another guy who was born without legs is perched on top of him so that it looks like one person pushing the table. Then the chainsaw "cuts" the top half off above the fake waist and sends the torso dude onto the table.
The reattachment scene was cut and edited, but not because they wanted to hide anything. They almost ALWAYS cut acts down on that show because they don't have time to show the whole thing on TV. Illusionists appearing on television perfect their acts so that they can stand up to repeated viewing on TV.
