Stop grabbing your opponent's head.
I thought he was trying to control the head to land elbows, similar to what GSP does with his can opener. That's the kind of thing the basic armbar is made for. The difference seems to be that GSP respects that armbar and lets go when his opponent changes his hips. I don't believe Henderson thought Pettis could catch him with something that simple. Over confident in his defense, maybe? Either way, it was beautiful.
What Pettis did that most people don't do is to control the angle of Henderson's wrist so that he was attacking the elbow the entire time. You can see him twisting the wrist from inside the lock.
Really basic setup, with really small details that made it work against a guy who NEVER gets submitted. It was awesome.
THE GUARD AIN'T DEAD