Once you have an underhook, where you put your hand and how you activate your arm is extremely important. The first thing you want to avoid is a floating hand, meaning your hand has to glue firmly to the back. This can mean simply pressuring your hand into the back to seal it in, or it can mean gripping the gi to create a stronger fixed point. The idea is to create friction between your hand and their back so your arm is harder to move and so you can more easily move them and move yourself around them. Ideally, I frame as high up as possible between the shoulderblades with my hand.
Once you fix your hand in place, then it becomes important to activate your arm. Most often, you can accomplish this by raising your elbow and sometimes your shoulder to shrug yourself to a more flanking position, which also limits the top player’s ability to form an effective whizzer and initiate brabo choke attempts.