Here’s John Danaher’s answer to that question:
Jiu Jitsu is a system based around 4 distinct steps:
1. Take your opponent to the ground. The closer someone is to the ground, the less they can employ explosive force. What’s the first thing cowboys do when they brand a steer? They lock up its legs and put it on the ground. Nobody tries to brand a standing steer, you’re going to get killed because it can employ dynamic, explosive movement. Taking someone to the ground takes away the single most dangerous aspect of fighting: Quick, dynamic movement that can generate explosive, kinetic energy. So step number one is get the fight to the ground. It’s inherently safer. Less things can go catastrophically wrong than in a standing position.
2. Get past the legs. If you are a skilled Jiu Jitsu player and I end up between your legs, you can armlock me, you can leglock me you can strangle me. Even if you are an untrained fighter you can up-kick me. Legs are dangerous. So step number two is to get past those dangerous legs.
3. Work your way through a hierarchy of positions. Once you get past the legs there is a sequence of pins. You’re going to go to knee on belly, you’re going to go to side control, you’re going to transition to mount, or transition to rear mount. Jiu Jitsu encourages you to go through those various pins. If you look at the sport of Jiu Jitsu, the pins are scored according to ones ability to strike an opponents and inflict damage. Knee on belly scores more than side control because you can strike with more power. Knee on belly is inherently unstable, however, so it scores less than mount, which is inherently stable and offers just as much punching power. So step number three is to work your way through a hierarchy of pins, where the pins are graded in value according to your ability to strike with effect on the ground.
4. Attack for a submission.
Jiu Jitsu is a four step system. It’s beautiful, it’s elegant, and it’s deadly effective.
And now the question needs to be asked, where do leglocks fit into that system?
They don’t. Leglocks don’t fit comfortably in that system. That’s why for generations, leglocks were considered a signal of failure. When you can’t take your opponent down, when you can’t pass his legs, when you can’t secure a dominant position, you go for a leglock. When you couldn’t get the system to work you resorted to a leglock.
What I tried to do is find an avenue where leglocks could come in, and the results were surprising. The four step process I described assumes a top position. The goal is to stay on top. Leglocks, however, assume a bottom position. 80% of leglock entries come from the bottom position or with an opponent is behind you. In other words, from supposedly inferior positions.
What I discovered is when you incorporate leglocks into the system you change the very nature of the sport. Jiu Jitsu, as it’s ordinarily practiced, is a single direction game—movement from the legs toward the head. If you get stuck or lose position, you start the process again. Traditionally it’s a mono-directional sport. But once you start adding leglocks into the game, Jiu Jitsu becomes a two directional sport.
So if I’m passing the guard and I can’t do it, I can fall back into the legs. If I’m in side control and my opponent recomposes his guard I can fall back into the legs. And I can play my opponent’s reactions in ways that open up opportunities for submissions more easily.