Apparently when something interests you, the best way to figure it out is to smack it really hard, and repeatedly. If you're a cat, at least.
I think hitting hard has to do with the perceived size of the thing. In contrast, a first encounter with a ping pong ball would be a light tap. A cat will tap a little puppy, but slug a big dog.
I'm tempted to buy a treadmill just to see how my cats will react.
Your YouTube link doesn't seem to work any more, but I think this is the same video: http://www.youtube.com/watch?v=OeYKqRRKpDo
Thanks to Laura (@2), the evil cats are back messing with the treadmill.
I saw a range of different touches - the poke and back off (which didn't look like a hard strike to begin with) is the first, which makes sense if they're expecting things to try to counterattack. I particularly liked the black-and-white cat trying to hold the belt back.
When in doubt, groom!