I have a love/scared-to-death-of-them relationship with leg locks. I really like kneebars and toe holds, I threaten them all of the time, and it makes my opponent scramble to defend. That will, typically, allow me to improve my current position. A good example of this is the rolling kneebar from the turtle position.
Using the knee bar as a sweeping tool if the guy stands up an breaks your guard:
and one of my all time favorites the kneebar from half guard:
Another really good one is the toe hold from omoplata sweep.
But the toe hold can be used against you if you attempt the omoplata:
The lock I am most scared of, and one that I almost never attempt is one of the simplest and most effective submissions there are. It is the heel hook.
As you can see, heel hooks are easy to set up and easy to execute. The problem with them is that they are a twisting knee submission, and you really don't feel like you are in danger until all of the knee ligaments snap. They are extraordinarily dangerous. So dangerous in fact that most Brazilian Jiu Jitsu tournaments do not allow them for any belts lower than brown belt. Even in the land of macho men and big egos, Brazil, the heel hook is a banned technique for those below brown belt. They are just too dangerous.
So, how do you train with heel hooks and not hurt anyone? Well, in active rolling, I like to go through the movements up until I get to actually hooking the heel. I will simply touch the heel and then let go of everything. I don't actually hook it or work the submission. More experiences guys will acknowledge the submission with a nod or, a good job, or whatever. Less experienced guys will wonder why I didn't finish it off.
It is important to recognize who you are rolling with when you attempt heel hooks. Often times, the one doing the leg lock will remind the opponent that they can do leg locks and you will start to trade them back and forth. If you attempt your heel hook on the gym spaz, you are going to walk away limping. Only work your heel hooks on guys you trust. The submission is very dangerous and the gym spaz will rip you knee apart when they go nuts trying to prove how much more of a man they are. Instead do them with the guy who is at class to have fun and to get a good work out in. He won't tear apart you knee, and will acknowledge your submission attempt.
There are many guys out there who are good at heel hooks. None, however, are as feared or as reviled as Rousimar Palhares.
The guy is a heel hook genius, gym spaz, meathead, and all around unstable crazy person. In his last three fights that he won in the UFC he has submitted his opponents with heel hooks... and in all three fights he didn't stop twisting until the knee was destroyed. What's worse was that he competed in the ADCC no-gi world championships in 2011, he advanced to the finals injuring all three of his opponents with heel hooks. He sent one of the guys, David Avellan, to the hospital for immediate knee reconstruction. Rafael Lovato Jr. limped away from the semi-final match but only suffered tissue damage.
It goes to show the immense damage that can be done by these kinds of submissions. I am too old to be messing around with knee reconstructions, so I am well known to tap out as people are setting up leg locks, rather than waiting until the lock is on and some guy is looking to tag my purple belt up on his submission wall. I don't care. I would rather have you bragging that you caught me in training than risk the cost of surgery, rehab, and go through all of the pain associated with everything.