The title of this post, it's a phrase I hear a lot. Usually in the form of a question, "What do you do if he's got legs?" My response is usually something like "So what? He's got two extra limbs to attack." That is somewhat of a snarky answer, but it's true. Asking that question is the same as asking, "How do you shower?" However unlike, "How do you shower?" It's not a dumb question just a poorly phrased question. Ironically, both of these questions have the same answer.
What you must understand is BJJ and every other martial art for that matter, is bio-mechanically perfect. They're all perfect at what they are specifically designed to do. Defend oneself under some given circumstance. The point is you'll never create "new Jiu-Jitsu" you'll only modify it to fit your level of strength, flexibility, agility, speed, etc. Ever wonder why people considered "innovators" like Ryan Hall, the Mendes brothers and the Maiyo brothers all finish people with what amounts to an armbar, leglock or choke? It's because every human works the exact same way bio-mechanically. They simply use their physical attributes to obtain dominant position in an unorthodox manner. An armbar is always an armbar no matter what direction the arm is pulled. A further example of this mindset is the saying "An omoplata is, but a kimura as is an americana." That is to say that all three submissions are the same. They are shoulder locks.
Now what does this have to do with whether or not someone has legs? The answer is I'd do the same thing you would do. I'd pass his guard and/or sweep him and then submit him just as you would. It'd be insane of me to think he wouldn't use his legs out of "fairness." Frankly, the idea of that alone is insulting. The real question how do I do that; and the answer to that question is I have to make his legs non-factors by implementing the "5 points of control" rule. Everyone has 5 major points of control on their bodies if I control 3 out of 5 I win 100% of the time. Those are the head, left arm, right arm, left leg and right leg. Now there are intermediate points of control such the hip, knee, elbow, etc. These are actually the points of contact that allow to control an entire limb. It's very rare you'll actually push an opponent's leg down and lay on it smashing the whole limb(useless you've of began studying flat passes, but that's it's own seminar and won't be discussed here) I hope that answers this question at least some levels.
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