This is a basic defense for the Key Lock or the Americana. Keep in mind that you can not wait to straighten out the arm, you need to do it as soon as possible. When you place that thumb in the armpit the hand is gently cupping the shoulder. Keep in mind that it is important to straighten the arm but please (PLEASE) keep in mind that the space is made with the hips, not with the arm.
If your opponent is truly trying to submit you and dominate that arm then it will be much easier to move the hips through that leg. By dominating that arm your opponents back half will become light (lighter in some cases with large individuals).
Once you get to the standing position it is important to have good driving pressure forward or you will find yourself in a scramble. The distancing, how you control your opponents legs, how you move your hips and the job of that arm will become much more apparent though repetitions. My suggestion is to rep this out a couple of hundred times before you risk trying it with someone who is trying to rip of your shoulders.
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