Stopping your opponent from a side-control using a technique of putting your thumb into your opponent’s  armpit will surely pave the way for you to escape.
(0:18) The opponent is now in control of your body and you are below him. The first thing to do is to stop the opponent by placing both arms against his left arm. The opponent will come up high on you and come over.  Your opponent will think they can just come over.
(0:31) So immediately, when the opponent goes over, you put your thumb right in his armpit. Can your opponent come around in north-south? Sure but you know what you’re stopping him by pressing against his hip.
And all you have to do is scoot out and then put him in the guard.
(1:00) So in this position, you take your thumb and just put it in your opponent’s armpit. Keith explains he saw this in the UFC the other day. The UFC guy, he put it there and they got up on their elbow and stood all the way up against  Roy Big Country Nelson. Okay he did it on him and just right there put it right in his opponent’s arm. Oh, he had discovered it.
(1:44) So when you’re in position and your opponent is trying to get over and he brings that arm over-the-top, that’s fantastic. I hurt Stead man and he’ll try to come back fine and he wants to come back on. Not allowing that.

Escape side mount with Keith Owen. Keith shows how to put your thumb on opponent’s armpit and scoot out of side control. This is something Marcelo Garcia also does a lot.