I once tapped a guy who weighed 280 lbs just by sitting on his stomach. Ass-on-belly. Essentially had him mounted, facing the opposite way, looking for a leg submission when he starts slapping me on the back.
I turn around and he gasps "TAP! TAP!"
I got off and was like ".... you sure?" "Yeah."
Weirdest submission I've gotten so far.
Thanks for the replies on my behalf, too. I've been caught with that a bunch.
Another one that I can add since you're doing no gi:
controlling knee on the belly of obviously much easier in gi (with grips and chokes available) than without gi. Often times you can slip out of a no gi knee on the belly.
That being said, if his pressure is off, you can try to elbow escape hard away from him (i.e. your stomach faces him) while you attack and wrap the leg thats on your stomach with your top arm (your top arm after escaping hard to your side). As Yrkoon pointed out, this will kinda get you to your knees/ turtling position, however now you have a leg that you can attempt to stand up with for a single leg takedown (and hopefully reverse the entire situation).
http://www.bullshido.net/forums/showthread.php?t=22295 ya ya search function
anyways, 2nd post by JohnnyS has worked wonders for me, also check out SM's post