with much heavier opponents i use that leg position (minus the lifting back and hips off the ground) in order to control distance and keep room to throw submissions. i have used that position you are talking about to bait a pass or leglock atempt in order to escape the ground.
A guy I was rolling with today used that sort of thing. It was a perfect opportunity to do a double-underhook pass since the feet are open and not hooked in whatsoever.
Going to knees on chest from closed guard can be fun and games if you maintain wrist control to prevent them from starting to pass or go for leglocks. There's a great triangle in 101 Submissions II where the guy holds both sleeves and puts his knees on their chest (with his feet on their hips) before snapping one of their arms out and kicking his leg over their shoulder for a triangle. It's when he needs to close the triangle that he shoots his hips up and straightens his back so only his shoulders are on the mat. I also train with a brown belt who loves to do this, but uses it to wind up to shoot his hips up for an omoplata.
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