Looks real to me right up to the shirt being lifted (look at the bottom of the shirt when it contacts his shoulder) as the cutting wheel hit it. It was running until the shady power strip he was plugged into dropped off the table. The force of the tool spinning up instantly caused it to twist and the blade to contact the table causing it to "jump" off the table. The reason you think it disappears is it rips the makeshift dust mask off of him and wraps it up so it's just a white blob as it drops to the floor. It might also be the reason this was not a much bloodier scene.