But what makes Rousey's armbar so dominant? What makes it so unstoppable?
It's a combination of things: some preparatory, some technical and some beyond the realm of either.
To begin, Rousey is ruthless in her preparation, a tireless athlete with Olympic-level qualifications and the work ethic to match. She may or may not be a one-trick pony, but it's irrelevant because she continually needs only one trick to secure victory.
She knows more ways to set up an armbar than most of us know words in our native language. She knows how to close the distance to force a clinch, how to get an opponent to the mat and how to snatch an arm when she's there.
Once she is, there simply isn't a more technically sound armbar in the business.
Whether she starts on top or rolls underneath to secure a position, Rousey is as close to flawless as there is. She has a thousand setups to isolate the arm once she's on it and keeps incredible pressure with her hips at all times.
The ability to isolate the arm from any ground position and adjust both her weight and the weight of her opponent is a crucial element of Rousey's success as well, as most defenses to an armbar involve some manipulation of one or the other. Most commonly, one would press their weight into the opponent to relieve pressure on their arm or stack the opponent's weight back onto them so the opponent cannot extend their hips and lock the arm in place.
With Rousey, she has perfect control of her own weight and an uncanny ability to manipulate the weight of her opponent, which renders many escapes essentially useless.
The other two contributing factors to Rousey's unprecedented submission success are more abstract but undeniably influential: experience and level of competition.