Often, when an opponent is playing situp guard, they will wrap themselves around one of your legs in a position similar to a single leg takedown. They can use this position to sweep you or wrestle up.
To pass this kind of guard, first establish an underhook on the arm the opponent is using to control your leg. If this is difficult, you can side step with the trapped leg to cut a small angle to dig in the underhook.
Next, change levels so you can use your head as a frame across the opponent’s face. Reach up with your free arm, leaning your opponent backwards.
As your opponent falls, maintain pressure with your head as a crossface and change angles with your trapped leg to initiate a knee cut/knee slice guard pass.
If executed correctly, you will have passed their guard with a far underhook, making it easy to pin them in side control.
A variation of this pass occurs when the opponent recognizes that their guard is being passed and brings up their free leg to attempt to get a butterfly hook. If this happens, knee slice between your opponent’s legs with your non-trapped leg, stapling it to the ground. From here it is relatively simple to use footwork to untangle your legs for the guard pass.
Another similar variation if you cannot secure the initial underhook for this pass is to cut across your opponent’s lap with your free leg. Your leg can be extended to wedge your trapped leg free