How to Defend the Armbar From the Back
Defending from the back has developed markedly in Brazilian jiu jitsu over the past two decades. Once someone got to the dreaded position, it was "check mate" for the other person. However, competitors and gym rollers alike evolved to protect their necks better and better over time, and as a result, back attacks have evolved and become more holistic in their attack approach, including the incredibly effective armbar from the back. Here's one very simple method, and a few key details on how to defend this attack as preemptively as possible.
The Nuts and Bolts
Your partner is about to armbar your right arm, and you know this because they have encapsulated your right arm in some fashion (possibly with a Kimura grip, but there are other ways). You know that the person must hip out to the side, then throw their leg over your head (to dramatically abbreviate the steps). The key here is to prevent that last step, not to try to let them do it and then work a classic armbar escape. Some people have found success in turning in toward their partner (to your right, if your right arm is trapped), but a good Kimura grip all but removes this chess piece from the board, as they can simply flare your elbow out, preventing the turn. Instead, bridge toward their hip, pinning your own shoulders to the mat, and then turn in toward your partner, looking to recover guard.
Even More Preemptive
After being caught in, say, a few dozen armbars from the back (and if you haven't been caught a few dozen times by the same submission, you're probably not rolling enough), you will begin to see the clear pattern at play. If your partner gets far enough so that they have the "kickstand" leg built up, you likely won't be able to prevent a savvy opponent from getting their leg over your head, as the kickstand acts to prevent the bridge escape. However, as they're working to transition to the position, it's easy to bridge very early, as soon as your partner's angle begins to change, but before they are able to settle into the position. As soon as your partner's grip passes over to the other side of your head, that is your trigger to bridge and change your own angle as close to perpendicular as you can get it. The ultimate goal of the bridge is to completely pin their thigh to the mat, making it all but impossible for your partner to finish an armbar. This leaves them with only one halfway decent option: to try to come up on top to mount. As they make this attempt, you have all of the guard recovery cards in your pocket. Use them.
On the Flip Side
Turning back to the perspective of the person doing the attack, it's important to anticipate this common reaction to the armbar from the back. As your partner begins to bridge over your hip, simply concede the point and allow them to do this, but turn your focus elsewhere. As they move to bridge, help them head in that direction by "flicking them off" with your foot, taking care to make sure your bottom leg retracts. You should end up with your chest on their back and with a Kimura grip (if you're using the Kimura grip in the first place). Now, you can simply reset and hit a nice technical mount armbar, which will be much more difficult for your partner to prevent.
Tougher to Escape from the Back
The armbar from the back isn't going anywhere. While it's true that it has been here all along, a part of both judo and BJJ canon for literal generations, it has only recently gained widespread popularity in both circles on the ground. Observing wider trends like this in the sport (and art!) is useful, as you can often extrapolate what is likely to happen with virtually any technique: it exists for a time in certain circles, then becomes widespread and mainstream (or "hot"), and then a counter is developed. Once the counter becomes its own technique, it is then practiced in certain circles, then becomes widespread and mainstream, and a way to defeat or deal with the counter arises, and so on. This is jiu jitsu's main source of evolution (although certainly not its only one), and these moves should be celebrated when they arise, not simply provide a new source of frustration for us. We are helping to create something amazing, each and every one of us. As always, please let me know if these techniques work for you!