Arm Bar - In the arm bar you are trying to hyperextend the elbow. You want to get your hips close to their body and turn the arm so that the elbow is hyperextended. This works great from the mount, guard, and back.
To defend, turn into your partner’s arm bar and grab both of your hands together and bend your arm. This will protect you only for a few seconds. Switch your hips and walk into the arm bar, stacking him so that he cannot extend his hips. Walk around to gain side control or go into your partner’s guard.