well yea it could still accelerate a little though right? because it still had all that force behind it and it hadn't been spent yet. It still gets to it's peak and then decelerates like it naturally would.
actually it starts decelerating, then goes slower and slower for a couple of rotations and then speeds up (does a rotation very slightly faster than those before it), after which it starts slowing down once again.

I'm still trying to figure out why they would mess with the muffler at all. Why would they need to spend the time and extra effort adding in a motor?
uh, maybe because it takes five minutes to do that and works well...
exactly. It doesn't need to be messed with. And if it did, it would have been much more effecient and less tiem consuming to just move the thing 6 inches closer to the muffler.
but that would decrease its cleverness, right?

(after all, they could just do a different ad idea that didn't require so much work; remember that it is an ad, not a science experiment, so there are probably all kinds of tricks involved to make it easier to do for them)