The guy on bottom wrapes his right arm around the arm just behind the elbow joint of the top guys left arm. Rolling the radial bone (forarm bone on thumb side) into the crook of the top guys elbow. The left hand of the bottom guy pushes left shoulder of the top guy away. Grip your left forarm with your right hand. You get bonus points for trapping the left hand in your armpit. While rolling the radial bone into the elbow push the left shoulder away. When you feel the top guy rise up bridge to your right. It will be impossible for him to stop the bridge with his hand since you have his arm trapped.
When training put your forarm on the tricept of the training partner and not on the elbow joint. Be aware that the top guy can push your left arm across your body and make it hard to move it back. This counter will happen while you are trying to push on the shoulder.
When you complete the bridge keep the armbar on. Arch your back while appling the lock. The joint lock is called a figure 4 armbar because your right arm grabs your left forarm making the figure 4 with your arms.
Careful and do this slow when you first practice this. Remember to stay off the elbow and put your arm behind the tricept instead while training.