Another way to think about this pass is to consider that you're almost always going to go in the following order:

Hips. You need to beat your partner's hips with your hips, and this means controlling (pinning) your partner's hips. Shoulders. Your partner's shoulders need to be flat on the mat, just like their hips. Head. Your partner's head must not be able to turn toward you.

While you may pass some guards some of the time without having all three of these characteristics, having all three helps to ensure that you will have a great chance of passing even a fantastic guard. Note that you can also do this guard pass without the underhook, provided your lapel grip helps to achieve these three objectives. Feel free to switch your hips back to a head and arm or "100 Kilos" side control position, unless you're comfortable with modified kesa gatame (in which case, you're good to hold right here).