I've been thinking about this a bit lately (most recently here). I did some specific with Chris after class last night, with me trying to pass his open guard.
One thing he kept catching me with was an extended hook. He'd hook my left leg with his right, full extended. Then, if I remember right, he'd reach down with his right hand and catch my pant leg by the ankle. After that, he'd pretty much reel me in.
I think I remember Mamazinho showing us an open guard pass technique that might help counter this hook and sweep. Ironically, in jiu jitsu's often bewildering simplicity, what I want to do is reach down and grab the ankle of the hooking leg with my hand from the opposite side (my right hand per the above example). With my same-side hand, I want to grab the sleeve on that side. If he's really going for the hook and sweep, then his sleeve might be easier to catch that I'd think.
You don't want to fight through the hook. So what you do is backstep with the unhooked leg. While you're doing this you want to pull on the sleeve toward you and pull the hooking leg out away from you. It's a lot like the spin pass from the butterfly guard post.
Give 'em a good, lawnmower yank on both ends, and then drop into knee-on-belly with the knee closest to the legs.
