i know exactly what you mean. The ankle of his trapped leg rotates in as far as it can go, so the whole shin and foot are basically flush with your ass, and it can be hard to get "inside" that foot to lockdown. There's a lot you can do, but honestly, the simplest answer is to hip escape. The farther out you are, the better the angle to weasel your outside foot in behind his knee. Then you can just make space for your inside leg to come through.
Or, keep in mind that when he does this, he is flaring his knee out a little. Hip out, stick in a butterfly hook with your free leg, and then get "perfect 1/2 guard" (underhook on near side, wrist or arm control on the other to block the cross face). This way, you can threaten a simple sweep (pull on far arm, elevate with butterfly hook). If he stops it by posting his far leg, you can take full guard or deep half. If he tries to ride it out with balance, he will basically pop up and then back down. While he is popping up and back down, you can either A) replace a full butterfly or closed guard, or B) open your legs real quick and basically let him drop into your lockdown, closing it as he does. I have a lot of luck with this. If you get good at it, you can even do it when the other guy has proper head and arm control instead of you.
Or, keep in mind that when he does this, he is flaring his knee out a little. Hip out, stick in a butterfly hook with your free leg, and then get "perfect 1/2 guard" (underhook on near side, wrist or arm control on the other to block the cross face). This way, you can threaten a simple sweep (pull on far arm, elevate with butterfly hook). If he stops it by posting his far leg, you can take full guard or deep half. If he tries to ride it out with balance, he will basically pop up and then back down. While he is popping up and back down, you can either A) replace a full butterfly or closed guard, or B) open your legs real quick and basically let him drop into your lockdown, closing it as he does. I have a lot of luck with this. If you get good at it, you can even do it when the other guy has proper head and arm control instead of you.
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