Combat base arises when you've gotten your opponent's guard open, and you have your knee up in the middle, keeping them from closing their guard. This is one of the best positions to attack with some very basic leglocks (and, of course, to pass the guard). You may have a great deal of success with either the basic straight ankle lock or the basic heel hook, but once you start refining your attacks and hitting some more advanced setups like the over/under heel hook, you'll really open a ton of doors.
This article will explore some options that are easy to get into, and it'll work well with other combinations you're likely to explore. If you're not sure if you're allowed to use heel hooks at your gym, be sure to speak with your instructor before attempting any of this stuff, even in drilling, as the positions are inherently more dangerous than many more commonly familiar ones! For info on how to open the closed guard, check out this tutorial.
Bread and Butter Heel Hook
Ah, the old classic: the heel hook from inside the guard. This opportunity often arises as soon as your partner's guard comes open, and they hip out to the side, just like they're going to try for a scissor sweep, just as an example. Nevertheless, this is far from a "gimme," as you need to be sure to first isolate their bottom leg with a "shin staple" or knee drive of your own. Once the bottom leg is isolated, then you can go about reaping the crap out of your opponent's knee, trapping the top leg, and ultimately, finishing a tight submission. Obviously, the heel hook is a devastating leglock attack, so please be sure to only ever use this under proper supervision, and with partners you trust completely. The heel hook can injure the ankle and the knee, and knee injuries can be cataclysmic.
When you set up the heel hook above, one issue is that, as you're setting it up, the person can roll out to escape. Although there are things you can do in combination if you anticipate their escape, or prevent it outright, it's still something that's much easier not to deal with in the first place, if at all possible.
Enter the over/under heel hook. This versatile attack can be hit from a wide variety of positions, but the combat base entry is the easiest to understand and start working with. Once your legs are locked into place, be sure not to let your opponent try to roll to escape, as this can actually cause your opponent's leg to blow out! With the over/under, you should have sufficient control to perform the submission extremely slowly and carefully.
Inverted Heel Hook
The inverted heel hook is a very natural follow-up to the basic over/under heel hook (which is an "outside heel hook" attack). When the person attempts to straighten their leg in order to prevent the initial over/under set up, you can easily switch the leg across to the inverted heel hook, perhaps the most devastating of all leg attacks.
Note: this same inverted heel hook can also lead to numerous other attacks and combinations as outlined in the tutorial on leglock attacks from the knee cut position, and even (in some cases) the same as the attacks from the bottom. Understanding the combinations and possibilities that will arise as your partner defends your initial attack is absolutely paramount.
I've used all of these setups and finishes numerous times in competition over the last 15 years or so. I was thrust into using heel hooks rather early on, as many of the rule sets of BJJ competitions did not outlaw heel hooks, for better or worse. As a result, I had many, many trials by fire, figuring out through injury, luck, and experience what worked and what didn't.
Note that the selfsame fancy and funky leglocks involve many of the exact same transitions and entries, albeit perhaps a bit more complex. The over/under position is the over/under position, no matter how you slice it. As previously mentioned, always use extreme caution any time you're performing leglocks, particularly heel hooks and other twisting leg lock attacks. Enjoy the moves, but be sure to use them in the right situations. As always, let me know if these work for you!