FWIW, I just always assume my mare will either jump an invisible fence, or suddenly do the "OMG Crouch". I tend to use a bastardized forward seat, because that helps when Mia jumps the fence I cannot see. I always try to keep my legs kinda wrapped around her. If she is about to bolt forward, I'll pull hard to the left (usually - her good side) and we'll do a 180. If need be, we may do 3-4 180s. It is far less common now, but she'll never be a horse who strolls down a trail, a cigarette hanging from her mouth...
I sometimes wonder what it would be like to be relaxed on a horse. But unless I someday stuff Mia, I may never find out.