More of a "trap" I think. He rolls over on his back like this all the time when playing. He'll then try to bite, swat, slap, grab...or a combination of all of those. Wiggling around and his little tail going 100mph. I think he's having fun. BUT, I also think it's somewhat of a defensive stance....where he has all four legs free to try to "getcha". Never seems upset though.
He just never growls or gets pissy. (unless food or a brand new toy is involved). LOL