you've probably seen it before. i had seen it a bunch before but never really tried it, just blowing it off as another rubber guard thing.
i'll use right and left to make it easier to understand:
-a guy is in your closed guard
-you get his right arm to the mat
-instead of taking an overhook, you connect your left knee to your left elbow above his arm
-you wrap your left arm under your left knee, thus establishing a semi-overhook on his right arm. your left elbow pit will be in your left knee pit
-wrap your right arm around his head and lock a gable grip with the left hand
-relock your guard
essentially you lock a really tight guard that eliminates all strikes. it feel just like playing the overhook game, but you don't have to get onto your right hip to attack. you can actually attack while your back is flat on the mat. this feels weird since it i've gotten so used to being offensive while on one hip, but whatever. from here all the same submissions are available as a normal overhook game. in order to free your right leg to pass his arm:
-keep the gable grip but swing your right elbow around the back of his head. drag the outer blade of your right forearm across his jaw until you can dig your elbow underneath his chin a bit.
-use the frame of your arms to pry his head upwards. this will bring his left shoulder higher into the air, thus allowing you to slip your leg out and wrap his neck.
from here you can immediately execute a teepee and tap him. of course the triangle is there as well, but go ahead and be and asshole and do the teepee. it's fun! if he pushes his head to the floor to try to make the teepee more difficult, swing your leg all the way over his head and finish the omoplata.
essentially you get the rubber guard feel without torquing your knees. my flexibility is not bad, but on big dudes i don't like yanking my ankles too hard. wonky stuff has happened in the past, and big dudes aren't afraid to yank really hard when you're in an awkward position.