Is an armbar possible from back body triangle

I tried it on my gf as you do and it seemed to work, but I'm wondering if it actually works and why you never see it. It seems like it works with an achilles grip (I think that's what you call it), get a high achilles grip around the forearm and lean back with their arm diagonal to their head. Is this move actually viable in sparring/ competition and if so what are good Set ups/opportunity's to use it?