It only took Ronda Rousey fourteen seconds to find her signature submission at UFC 184 against #1 contender Cat Zingano in the fastest finish in UFC Championship fight history. However the armbar was not the traditional armbar that MMA fans are used to seeing. It has many names, though it is sometimes called the "wrong way armbar".
While the position is a bit unusual, it works on the same principle of all other straight armbars, extend the elbow to the point of hyper-extension and beyond, the only thing that is different is the orientation of Ronda's body in relation to the arm.
For a quick demo of the armbar in the context of a headlock escape, here is BJJ Blackbelt Vinicius "Draculino" Magalhães, trainer of several BJJ world champions and all around bad man.
So now that you have a feel for it let's look at Ronda's armbar.
It all started after that excellent reversal, Cat seemed to get a bit lost in the transition and left her arm wrapped around Ronda's head, which is pictured below.
Above you can see Cat left her arm hanging out, leaving her badly exposed. Ronda has clearly recognized this already as she has stepped her right leg over Cat's body to start attacking the armbar.
Above you can see how Ronda has reached back to rest her right hand on Cat's back, trapping Cat's arm in Ronda's armpit and from here Ronda is in excellent position to attack the arm. At this point Cat has almost no good options, she could attempt to spin hard to the left but that only might have resulted in Ronda finishing the armbar from her back and not belly down.
Ronda steps her left leg across Cat's face and sits to face Cat's legs and starts to lean back, putting intense pressure on Cat's arm from this position. This style of armbar brings pain and damage very quickly because Ronda pushing her hips forward transfers a lot of force to the arm.
It was an outstanding piece of transitional grappling. If you enjoyed this quick breakdown tune into my podcast Inside the Guard this week for a more in-depth breakdown of the throw and reversal that lead to the armbar, where Ronda's years of Judo came into play.