Ever see a MMA fight where you see the fighter who is pulling half guard suddenly turn into the opponent and shoot his hand under the non-trapped leg? It's because that person is either working a sweep, or looking for a kneebar. While initially, if you're not experienced, it looks impossible to pull off a kneebar from such a position, watch the video above. This setup is becoming more and more popular as Sambo grows.
Since I'm short on time, I'll just explain the first technique shown, as it is the base for the variations shown after.
1. You're in half guard, and you managed to squeeze the outside knee in to prop your opponent up on it.
2. Shoot your arm under the non-trapped leg of your opponent and pull, while pushing off the body of your opponent with your knee, which should create enough leverage to knock him over.
3. Throw your outside leg over the body, while bringing the inside leg deeper and up to lock up the legs and finish the kneebar.