Rafael Barata Freitas is one of the best rooster weights in the world. He is famous for his energy filled style and for his famous “Barato plata”. It is a very powerful shoulder lock.
If you get the mount and you find that your opponent is keeping his arms really tight so there isn’t any space to work on the collar, Freitas has a technique that is very effective. In this situation Freitas explains that you start by attacking the arm that is on the bottom. Reach between his elbow and try and hold off the same hand so you can trap both of his arms in one place with one hand. The other hand will hold his elbow down so you can step up with your leg. The hand on the elbow creates space so you can step over his arm.
At this time you can let go of his hand to control your leg. Now you can put your hand on the ground, adjust your body, and use your hip to move your opponents wrist to the outside. This creates a lot of pressure on the shoulder which should have your opponent tapping very quickly: