The North South Arm Bar is another great submission from this position. In the video above he starts the technique from Side Mount.
From NS with Head & Arm control you can still work this technique. You transfer your arm that is under his neck to trap his elbow. After trapping his elbow you cut your forearm deep under his arm palm facing up until you get your wrist to his armpit.
Once you have his arm trapped under his elbow you can finish the tech the same way as the guy in the video does.
One thing that I would do once you have the arm stretched out is to cross the leg that is at his armpit over his chest. This will help to maintain control if your opponent puts up a fight.
The hook that he uses to trap the arm is correct. You want the arm that is closest to his head to trap his arm. This will allow you to control his legs with your other arm.
If you trap his arm with your other arm then you give your opponent an opening to get to his knees where he can use his body weight to defend the arm bar.
