From inside the guard.
The trapped arm: Never cross over the mid-way of the guy's chest. Instead, grab the collar on the same side.
The free arm: Use this arm to block the leg that is trying to trap your head. That leg might come up quickly, so be ready to block the leg as soon as you feel the guy's weight shifting.
The block: Your hand blocks the guy's leg in a way similar to the hand position behind the head in the rear naked choke. You are blocking the second leg, the one that will go across your neck. Your hand should be between your face and his calf.
The legs: One knee up and one knee down.
The knee up should be on the same side as the trapped arm. This is also good as a general rule any time an arm is trapped. Move to crowd the guy on the same side that he is trapping, jamming him.
Bring the up knee around and down into his belly as you stack him on his neck and begin to work your arm free.
The knee down should be perpendicular to the "knee up" leg and tight on the guy's butt--under it is even better.
The stack: Drive the knee up into the abdomen as you rotate around his butt toward the side of the trapped arm.
Stay Tight! The more you extend you body, the more the guy can extend his legs and execute the armbar.
Drive the knee down under his hips as if to prop him up or support his hips. This will help stack him without the need to stand up.
From the full stack: You can either stack and pull your arm out, continuing to work around to the side until you have passed his guard.
OR
You can bring the down knee around to the side and move the knee up (the one that was grinding into the abdomen) over the guy's head. Here, you want to straighten that leg out and swing it over the guy's head so that you are essentially sitting on the guy's head.
The armbar threat will be eliminated and his guard will be passed. You will either wind up in north-south or side control.
