Ok for this shot you need a double sided knife. It needs to be VERY sharp.
The trick is to set a really slow shutter speed so that the knife (and hand) isnt seen and set the flash (off camera to the left) to full power which will freeze the action before the pieces fall but the slow shutter speed (about 10 seconds) makes the fast movement of the slicing invisible.
It took me five bananas before I got this effect.
I definitely didnt use any cocktail sticks or do any post production work in Photoshop. Honest.