This thread is for questions that you have like this, ones that you think are just rudimentary and basic. That's the purpose of this thread. Trust me, it's not annoying, it's why I did this thread.
So, just for reference, here's a clip of the Fedor vs. Choi fight, so you can see what I'm talking about and watch it a couple of times.
Really, this is a simple armbar done very, very quickly. In fact, this is the most traditional I think I've ever seen Fedor's BJJ.
So, to start this armbar, trap the arm. (Fedor traps the wrist by catching a punch, but however you intend to do it, you need to have control of that arm)
Fedor then puts on foot on the hip (he steps on the outside, I tend to step on the inside because I think it's more stable, but as long as you have some stability), then he spins, swinging his outside leg up and around and putting that over his opponent's head.
Make sure that you get your hips up in the air, so that they land above the elbow. Fedor does this really, really well and gets almost as high as the shoulder, which is the ideal position. It becomes harder and harder to finish the lower you are on the arm, and if it's below the elbow, then there's really no finish.
Really, this is a basic move performed very, very quickly. I would practice that hip movement slowly and alot. I do it in gi grappling by grabbing my opponents wrists, putting my feet on his hips (like a close spider guard on a kneeling opponent), then I spin my hips out so that I'm up on the very top of my back (right below the base of the head).
knew he could explain it better than I haha