The principle of concentration of force
One of the founding principles of all modes of combat, jiu jitsu included, is that of CONCENTRATION OF FORCE. The idea is that I want to take a large percentage of my total strength and apply it upon a a specific target that (A) has a high kinesiological value - if it were damaged the opponent would find if very difficult or even impossible to continue fighting, and (B) represents a small percentage of my opponents total strength. So for example, in the case of a well applied ashi garami, both my legs, my entire posterior and anterior chains of force and both arms are committed against one of my opponents legs. I don’t care how much bigger and stronger my opponent may be than me - it will never be the case that one of his legs is stronger than almost my entire body (if it is, then you should be using large caliber handguns against him, not heel hooks!) The main target of the attack, the knee, obviously has great kinesiological value - once broken it is very difficult indeed to keep fighting effectively - at the very least your combat effectiveness will be severely degraded once it is broken. ANY COMBAT METHOD WHICH MAKES CLAIMS TO EFFICIENCY AND AN ABILITY TO OVERCOME BIGGER AND STRONGER OPPONENTS, MUST MAKE USE OF THE PRINCIPLE OF CONCENTRATION OF FORCE. As a student of jiu jitsu YOU MUST MAKE A LIFELONG COMMITMENT TO THE PRINCIPLE AND A DEEP STUDY OF HOW TO APPLY IT IN PRACTICAL WAYS. The first step on the martial arts journey is recognition of this principle - every subsequent step is simply a further articulation of it. Recognize it and live by it.