It only took me a few minutes of studying the snapshot of Jean Jacques Machado working half-guard against Dean Lister's pass to see a few things that I've been ranting and raving about when it comes to the half guard.
The pass that Lister uses is really a guard opener. Jean Jacques basically has Lister's leg figure-foured, and Lister is using the leverage of his outside leg to press down on the locking leg, Jean Jacques' inside leg.
The problem is one that I've been spotting in half-guard work: relying on the inside leg to keep the guy's leg trapped.
The only time this seems like a good idea is if you are going for the Ichiro sweep or the Jeff Glover take-the-back move (the Glover Split?). Otherwise, you want to use your outside leg, with your foot pressed down or near the mat, to trap the leg, and use your inside leg to either come up on that knee (for a tackle sweep) or to use to help escape your hip out for a twist sweep.
Even then you've got to be careful not to get flattened, which is far more likely if you are using your inside leg to trap the half guard rather than the outside leg. Using the outside leg makes it much easier to take-the-back, tackle or twist, but most importantly, it helps keep you on your hip and better able to react to changes in the movement from the guy in your half guard.
