Defense against a modified armbar
One of my main training partners is a very good and very athletic brown belt who consistently is able to finish armbars started from either top or side on me. This is frustrating because besides him I am very rarely armbared. This is in large part because I do a very good job sitting and stacking out of armbars, so much so that I often bait armbars even against browns to get out from under side (many don't take me up on it anymore).
My training partner, who we'll call "J", has completely shut down my normal defenses by a rather simple change to how he armbars:
Instead of sitting back for the armbar, he goes to his side (the side facing my head). If J is armbaring my left arm, he'll be on my left while sitting/laying on his right side. Inexplicably (well, kinda, I generally get why), this does a far better job keeping me from sitting up even than hooking the leg. This method basically keeps me flat on my back and so then it is just a matter of J breaking my grip.
Has anyone come across this? If so, how do you deal with it?
Answers of "not getting put there" are not welcome.