Gracie Barra Bristol, (BJJ), Can Sönmez, Bristol, UK - 23/08/2012
Before I start, I just wanted to congratulate two old training partners of mine, Yas Wilson and Sahid Khamlichi. Yas has been tearing up the female competition scene for the last couple of years, establishing herself as among the best women in the UK. She even fought at the ADCC last year. There's no question she's at the brown belt level. It's also really cool to see more higher ranked women in this country - we're up to two black belt women in the UK so far, IIRC, a group Yas will no doubt be joining in a year or two. :)
Sahid got his purple belt the same day I did. However, we are at completely opposite ends of the spectrum: he had been smashing people at every competition he entered and then went on to win yet more medals at his new belt shortly after receiving it, whereas as I felt (and still feel) like a mediocre blue belt. So, I'm not at all surprised to see that a little over a year after getting his purple, Sahid is now a Roger Gracie brown belt. Very well deserved! :D
In the back position, your opponent is frequently going to begin by trying to attack your neck. Protecting your neck is therefore a priority. In order to choke you, they need to block off both sides of your neck. That will normally use either your gi (e.g., sliding choke), their gi (e.g., ezequiel choke), your arms (e.g., arm triangle) or their arms (e.g., rear naked choke). Therefore you have to be aware of all four of them: note their grips, if they're trying to pull their gi across, if they're attempting to thread an arm through yours, and most obviously, if they are attempting to drive their arm under your chin.
As with any escape, you need to stay tight. Keep your elbows in, using your hands to cover your neck. There are numerous schools of thought on just how to do that: clamping your hands to both sides of your neck (which I learned as the 'Shirley Temple' defence), crossing your hands over your neck, grabbing both your collars, and Saulo's method of just grabbing one collar, keeping the other hand free to block.
Saulo's back escape starts by putting a thumb inside your opposite collar, using your other hand to block their hands. You then do what Saulo calls a 'big scoop', shifting your upper body down and your hips forwards. Next, kick out one of your legs to clear their hook (you may also need to nudge it with your elbow), then drop your other elbow down past their other leg and turn. You need to be careful here that they can't re-establish their second hook: block it with your elbow and knee if they try.
If you're a bit late and they've already got an arm across your neck, fall towards the open side, as if you were reclining on a couch (if you fall the other way, you're helping them get the choke. You also want to turn your head towards their elbow to relieve pressure. Both Saulo and his brother Xande suggest that when you fall to the side, you want to be lying on their knee (Saulo suggests just below the knee), as that will stop them shifting to mount or re-establishing their back control. From there, Xande adds the detail of turning your hips to clear their hook.
Step your leg over, using that as a base to shrimp out. Grab their other trouser leg, to prevent them from moving through to mount as you try to escape. Keep shrimping in order to clear their leg, aiming to either re-establish guard, or continue to shift your hips back into their armpit until you can switch to side control. Make sure that you are still being careful of your neck, as that is always a danger from back mount. Saulo mentions that you could use your free arm to stop them sneaking their other arm around, though generally when escaping the back, he emphasises that it is your hands that do the defensive work rather than your arms.