You’re not alone if you don’t know what a calf crusher is because it’s a very under-utilized and under rated submissions. Some technical instructors of BJJ would call this a “Knee Compression Lock”. In the wrestling circles it’s variations are known as the “Calf Slicer”. It’s a super ruthless move intent on quickly destroying your opponents knee.
One of the problems with submissions involving the ankles or knees is that you don’t feel any pain until it’s too late. The pain that you would normally feel with chokes or submission involving the arm isn’t there in these submissions so they’re very dangerous to practice. What I mean is, before you’re ready to “tap” while practicing the move, the damage can be done without warning. The motto is “If you feel pain, it’s too late”
For street defense, the object would be to execute the move to it’s full range of motion. This would dislocate the knee, tear ligaments, and cause tendon and soft tissue damage. This is not something you want to get caught in.
Here’s a video of a training class where they are practicing submissions off of an arm drag. I personally think is one of the best setups for attempting the calf crusher submission:
Below is an example of the move being utilized quickly in a professional fight. I’m aware that the calf crusher isn’t allowed in every competition, but in the video below Ben Pittsley won the submission of the night.