In a standard 10 minute intro class under the Gracie family, the first move that is taught is the trap and roll escape. Within this lesson, a key point is made that the person mounted on top of their opponent has a clear advantage when it comes to gravity and the relative amount of power they can generate with a punch in comparison to their adversary. Under the same pretext, they teach punch block defense skills from the bottom of the fight that enable them to soften the threat of being knocked out.
In this video, the Gracie brothers attempt to quantify the intensity of punches by applying a numerical value based on their mode of application. A wrist punch is a 1 out of 10 intensity, a punch that has a wrist and an elbow behind it is 2 out of ten, a wrist/elbow/shoulder would be a 3 out of ten. Once the hips can be added to the equation, throwing your body weight behind it, it becomes 4 out of 10. A 5 out of 10 is the same as a four except that it includes gravity in the equation. A six through ten intensity is variable based on who the person is in fact throwing the punch.
Whatever the scenario, there can be no doubt that by engaging in a clinch-focused defensive posture you minimize the potential for damage by taking as many limbs as possible of your opponent out of the equation.
Watch further as the brothers break down their viewpoint.