Andrew Smith is a 3rd-degree BJJ black belt based out of Richmond, VA (Revolution BJJ). He runs BJJ Path, a video tutorial website.
Made to Be Broken
The mantra "position before submission" is a great rule of thumb when you first get started doing jiu jitsu, but after a few years, some of the rules in question can be bent or even broken. Side control is an especially dominant position, and giving it up to go after a leglock might seem like a silly thing to do, but under the right circumstances, leglocks from the top of side control can be a very effective attack. Further, they can often catch a very savvy opponent off guard. Here are some simple and not-so-simple attacks you can use from the top of side control. As always, make sure you understand the rules of your gym before trying to go for these while rolling. With proper caution, these can definitely be integrated into your game.
The Toe Hold
This is the simplest attack, and one I've been using since my days at white belt (yes, toe holds were once allowed at most local tournaments for all belt levels). Start with a solid side control, and let's assume that your partner is putting their right foot on their left knee in order to prevent you from moving to the mount. This couldn't be any better for the toe hold. Reach around your partner's ankle with your right arm, and simply grab your right wrist with your left hand (thumbless grip is ideal here). Once you've caught the submission, simply bring their foot in close to their body and torque outward (and for a review on how to finish a toe hold, check out these finishes).
The kneebar is another solid, simple, effective attack. From the same side control used previously, slide your left shin across your partner's hips. Now, crouch on your other foot, allowing your knees to be at a roughly even elevation. Lean away from your partner while you cup your own leg with your right arm, trapping their foot and ankle behind your armpit. Step through with your right foot onto their far leg. Finally, you can sit back (carefully!) and finish the kneebar. For a review on some fundamental kneebar finishes, go here.
Inverted Heel Hook
Once again, our partner has been kind enough to put their foot up on their knee while you have side control. This time, thread your right knee through the triangle of space their leg has made for you. Hug their knee tight to your chest, avoiding any space creation during the next step. Once again, crouch so that your knees are roughly at the same height from the ground. Post with your left hand if you need to, but don't let go of their knee with your right hand until you can pinch your knees together. Note: in this video, I use my foot on Vince's hip to finish a classic heel hook, but the over/under heel hook is also right there, and likely a preferred finish for most.
This position starts with reverse kesa gatame, or "twister side control." With your right arm draped over your partner's chest (but underneath their arms, unlike a Kimura set up), start by pressuring upward, making sure your partner can't recover guard immediately. Push their knees away, and when they push back, help them go a little further than they originally intended by pulling. Now, hook their right leg with your leg, making sure your calf is perpendicular to theirs (much, much more detail on this in the ninja roll tutorial). Establish a lockdown, keep your knee open, and roll through until you have the calf slicer submission right in front of you. In a perfect world, try to grab your partner's toes, not their ankle, and finish by pushing outward on their knee.
The vaporizer is another favorite of mine, covered in more detail here. Start by lacing your leg all the way through your partner's legs from a traditional side control ("head and arm" is a common name for this from of control). Drive your right knee to the ground, twisting your partner's legs up. Now, establish a lockdown on this leg. The rest of the move is virtually identical to the rolling calf slicer from earlier, except that you have their other leg trapped, and this submission is way more unpredictable (and potentially more brutal, so use caution).
That's a Wrap
I started learning leglocks way, way before I really should have, at least to the extent that the rest of my jiu jitsu game was deficient. As a result, I tend to have an eye for the unorthodox. These leg submissions are all attacks I've used successfully over the years, and I consider them extremely effective techniques for use against someone with extremely good upper body defenses (other black belts in particular, but not exclusively). As always, let me know how these techniques work for you!