Have been breaking down Cross-Sides Top, and a very simple observation made yesterday is when the bottom player is able to get to his/her side, they kill your ability to nullify shrimping with an underhook.
On the other hand, when flat, a bottom player can still shrimp when underhooked, but their shrimp range is extremely limited plus they’re carrying all your body weight. Hence, they’re easy to track.
In terms of solutions, provided your training partner doesn’t have a frame, you can flatten them back down by manipulating the spine via the chin/neck, just understand the tracking power of an underhook is significantly reduced if they get to their side, and even more so if they get a frame or underhook.
Roger Gracie countering Mario Sperry getting to his side with an underhook of his own with head chancery/chinstrap and leg weave @ 10:06-10:11: