Attacking Legs

A leg and an arm function very similarly as the joint setup is basically the same. 
Hip/shoulder
knee/elbow
calf/bicep
ankle/wrist
So in much the same way there are only a handful of actual leg submissions, and understanding how a leg functions will allow you to better navigate the nuances of individual leg positions.
1. Hip Locks- we will include banana splits and other attacks of the thigh in this category. There are very few true attacks on the hip joint, but they do exist(I have absolutely seen someone get their leg kimuraed). We do however see many attacks splitting the legs in the 10th planet system, and given the legality of such attacks across all belt levels 13 and up, you would think it would be more prevalent. 
2. Leg Hyperextension- functions the same as the arm hyperextension with the heel serving as the thumb to designate direction of pressure. 
Calf Compression- compression locks occur when we take the top and bottom of an extremity and squeeze them together over a sharp frame. In compression submissions like a bicep or calf slicer, we take our opponents bicep/hamstring and connect it to their forearm/calf with our shin or forearm sandwiched between, a lot of the time it is very painful and occasionally it will snap a forearm.
3. Straight ankle- Guillotine the foot, that's how I look at a straight ankle lock. Both attack and defense have very similar mechanics to the easiest choke in grappling. It's legal across all belt levels and if you get good at it it's very frustrating to deal with.
4. Toe holds- You can basically hit a toe hold from anywhere, the most similar to a wrist lock on the lower body, you don't even have to be in a good position, but it is preferred.
5. Heel Hooks- Inside and outside heel hooks function the same, toes bent under the armpit, opponent's knee is bent not straight, control the hip so as you twist they don't. There's a lot of nuances to a really good heel hook, but most people will tap before you get the chance if you do it right.