Belly-Down, Straight-Ankle Lock
Belly Down Footlock
The belly-down straight ankle lock has amazing leverage, and isn't necessarily respected as well as it really should be. Best of all, it works extremely well in conjunction with a knee cut guard pass. If you haven't already done so, please refresh your straight ankle lock fundamentals, because this tutorial will contain some sneaky stuff (and some key details that can help you get the submission!).
Start with a simple knee cut guard pass. Once your partner starts to defend the pass, change the direction and head toward their feet, making sure to create a "shelf" with your thigh and armpit (more details on this below). Base on your head so that you can have a better leverage point, and also so that you can free up your hands to make a full figure four grip (make sure to push outward here; more details below). Bring your feet together (think: prayer feet), and, while basing on your head, slowly widen your knees so that you can get the submission.
The Set Up
The cue you're looking for is for your partner's hips to switch from guard so that one leg is lower than the other. Your goal is to staple this lower leg with a knee cut pass maneuver, but also to create that shelf we mentioned earlier. "Kick" your partner right behind the knee, helping to ensure that it stays bent, and then scoop their foot up onto your hip by propping your free (non knee-cutting) leg up like a kickstand on a bicycle. Use your armpit and triceps area to keep the foot trapped in place. The more you can make this into one clean motion, the higher the chance you will catch your partner's foot here.
Making the Grip and Finishing
Once you've cut your knee across to catch their foot, gotten your triceps and propping leg to successfully work together to keep the foot trapped, there is still the submission itself to be executed. Like virtually every submission in Brazilian Jiu Jitsu, the set up is more than 90% of the work, and a good set up will yield a good submission most of the time. Here, there are a few smaller details that will help. First, be sure to put the flat part of your wrist (the top of your forearm) flush with the back of their ankle, running right along the Achilles tendon, but still flat. Next up, push their shin outward with a fair amount of force; this rotational energy will be converted into foot-breaking awesomeness if you keep things tight here. Keeping the pressure on, make your figure four grip by grabbing your own wrist (your fingertips should be slightly white here, indicating a lot of built up pressure). Make sure to close both elbows tightly into your ribs, making sure there isn't any extra space. Plant your head (you can slow this down so your head doesn't make a loud "thud!" sound by keeping your propping leg planted for now), and then bring your feet together. The general concept is to widen your knees so that your hips can lower to the ground, creating a great deal of pressure right on the fulcrum of the lever you've created behind their ankle.
Belly down armbar or footlock?
The Upside Down
Belly down submissions have a way of looking pretty flashy, but generally they're just extensions of the original submission you probably learned as a white belt. After all, jiu jitsu works just fine upside down; it's just a matter of tweaking some details so that the move now works in the confines of a "belly down" state. This means that you can widen your range of submissions simply by considering different orientations, and this will open up your ability even more to be creative while training. Always err on the side of safety whenever experimenting, but don't fall into the trap of fear -- instead, always make sure you have an open mind and a good attitude during training. As always, please let me know how these moves end up working out for you!