Compression locks (a.k.a. muscle lock, muscle crusher or muscle slicer) refers to a submission hold that is executed by pressing one of the muscles on the body against a bone, which causes intense pain and the submission. If the opponent does not submit and/or the technique is applied with considerable force, the muscle and/or tendon being compressed may suffer a tear. The most common versions of this kind of submission are the achilles lock, calf slicer and bicep slicer.
Although compression holds such as the bicep slicer and calf slicer are illegal in many BJJ tournaments, those techniques and others like them do remain legal in mixed martial arts competition. But just because they are allowed does not mean that you see them very often. In fact with the exception of the achilles lock, they are very rare. This could be due to the difficulty in setting them up or because once the practitioner and opponent have become sweaty it is easy to slip out of them.
If you are looking to slap this lock on your opponent, it is recommended that you attempt it early in the fight when both you and your opponent are relatively dry. Once it is past that point it would be smart to stick to simple submissions that are hard to slip out of, such as the rear naked choke. This is not to say that you should pass up an opportunity if it presents itself. If you think you can get it, go for it.
Like learning any submission, practice is the key. So try catching your partner with a compression lock the next time you roll.