FRAT warning (but it's a good post, dammit...):
Interesting: I said in my first post that two of my gym's best purple belts used the half butterfly guard. Now, one of them said that recently he's using the knee across half guard more, just like me! Cool :)
(Actually the knee across half guard is sometimes called the Z guard so I think I'm going to start calling it that, with or without an overhook (which some guys like from there, but not me, so far).)
I was rolling against this guy an hour ago and changed my strategy a bit: To "attach" myself from guard bottom, I either slid right into Z guard, or grabbed his wrist or sleeve with one or both hands and put my foot on the same side on his hip, to work some basic open guard moves. (Z guard also worked against him standing, FYI.)
I felt that this worked MUCH better for me than to sit in the butterfly position like Marcelo Garcia, trying to get my butterfly hooks in place. For me, at least. And the purple belt told me that my half guard game was pretty good :)
We also did several rounds or "isolation" where he allowed me to start in the "oldschool" position, or very close to it and to my surprise, I found out that my game from there, which I had stupidly thrown away in search for something else (mostly open guard and butterfly guard), can even give very good purples like him a very hard time, IF he allows me to get that deep under him. I was shocked to find out that it wasn't until he was resisting like a MADMAN, competition-style, that he could SOMETIMES stop me from sweeping him - again, IF he allowed me to get deep under him.
I also found out, by thinking for myself (during working hours ;) ), a great way to get from Z guard to the oldschool position! I could always get Z guard and my oldschool sweep was strong, but I always had a hard time making the transition between the two. It was because I was "diving" underneath my opponent, leading with the wrong hand. My original way was to remove the knee in the chest and dive for that underhook, then try to grab his free foot with my other arm for oldschool. What would happen is that before I got my underhook deep enough, I got squashed with a shallow underhook - not good.
But a MUCH better way is to do it very similar to how Saulo Ribeiro teaches it on his instructionals: FIRST dive with your non-underhooking arm to either grab his free foot or underhook his leg. THEN (AFTER you've gotten underneath his stomach) you take out the knee in the chest and pummel for the underhook (very low around his thigh, actually) ... and work for the oldschool sweep or related sweeps.