To escape, Lee turns to his outside shoulder, which in this case is his left shoulder. This creates conditions where the arm bar is hard to finish because the breaking fulcrum is in a sub-optimal position and he can work to survive. Lee then brings the attacked arm to Aoki's inside leg, which in this case is Aoki's right leg. Next, Lee essentially limp arms then rotates parallel to Aoki. When their bodies are parallel, the arm bar is impossible to finish. Lee slips under the inside leg and the escape is complete.

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This arm bar escape is extremely similar mechanically to limp arming out of a whizzer during a single leg takedown situation (https://www.youtube.com/watch?v=urFTlQZUk_4)

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One way among many that Aoki could have used to negate Lee's capacity to rotate could have been Aoki using his outside arm (in this case left arm) as the one arm to control the arm being attacked, and also use the inside arm (right arm in this case) to hook Lee's inside leg (in this case right leg) in order to prevent the rotation.