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