In Brazilian Jiu Jitsu, one of the most paramount parts of learning is being able to develop a systematic approach. Think about your first few bjj classes and what you learned. Let's take the closed guard for instance.
In closed guard you are taught a sequence of moves so that you can chain attacks together safely. For example, you may learn the arm bar from closed guard, if they pull their arm out, you go to a triangle, if they get their head out, maybe an omoplata. you are learning a system. In this system there are sub-systems and secondary options where you may be able to implement a sweep or something.
When people are introduced to leglocks, they are not being taught systems because so many people neglect them. Many of us go to our academy and we are randomly taught a heel hook or ankle lock from outside ashi or some other leg entanglement. We have no idea what reactions we may get or how to safely enter these submissions.
What is the outcome?
Now when we are rolling, the class spaz decides to try these newly learned submissions with no control and no system and he hurts your leg. This may have happened because you did not know the mechanics, the escape, or because he did not have a system and did not know how to properly enter and apply this attack without hurting you.
There is a HUGE difference between pressure and pain.
That is why you need a system for your lower body attacks!