The purpose of this drill is to develop a constant effort to get to your side in half guard as well as to help you develop the physical strength to get to your side. Since our shoulders and hips are attached to each other by a moving sea of muscle, bone, and other stuff, often you will feel pretty fatigued trying to connect these two parts of your body against a resisting opponent. This drill will help a lot. The idea is for the top player to flatten the bottom player out using their body weight and for the bottom player to fight to get back to their side. Really, this becomes a game of the top player looking to put the bottom players far shoulder to the mat and the bottom player looking to take it off the mat.
As far as resistance goes, I prefer to focus the drill on one person or the other trying to improve, and I coach the other person to “challenge but not frustrate.” I want them to provide enough resistance to make it hard for the other person to accomplish their goal but not make it impossible. The idea is to provide challenge to develop skill. Once the person improves, then the other player increases the level of challenge. I usually set the challenge at being hard enough where the person is successful 7 out of 10 times.