The Helicopter Arm Bar is a nice flashy move that works great on newbies and is an excellent technique for demonstrations. There are also some nice follow-ups for this technique when used against better grapplers.
The technique is broken up step by step down below, here is a video of Eddie and Chaz demonstrating the technique:
I start in the 'Feet on Hips' guard position on one of my Junior instructors, Caleb.
Keeping Caleb's weight on my feet I lean back and hold Caleb above my head with my legs.
I extend my right leg towards the celing as I take my left foot off Caleb's hips, This motion forces Caleb's right hip to fall downward. I also pull down on Caleb's right arm to assist in this turning motion.
As Caleb begins to fall to his right side, I begin to circle my left foot around Caleb's head. At the same time I spin counter clockwise on my shoulder to get a better angle on Caleb's head.
As Caleb Lands, I release my grip on Caleb's left wrist and lock both hands on Caleb's right arm as my legs are in the arm bar position.
I squeeze my knees together and finish the armbar.