I also normally have fantastic spatial awareness! I think for me it comes down to the ‘purpose’ of the movement or what would get that 9 or 10 score in the ring. In haunches in, I think about moving the hindquarters with just my leg and seat, independent of the front end. In the half pass I work on the reach and the angle, both of which involve the shoulder, and therefore my upper leg. That’s what I mean by using my thigh to open or close the door. It’s just a visualization tool which may or may not work for different people.
Great point about separation of the aids. We talk constantly about the independence of the aids. It’s been really helpful in my teaching to have students understand which parts of their body and which aids support or control what parts of the horse. We don’t talk about it a lot in lessons, but it helps them and me figure stuff out on our own or identify which part of our riding or weakness in the body is causing the problem. Dressage is so fun because you can pick apart the technical but also blend it with more abstract ideas, i.e., my aids for the haunches in and the canter are similar, but my horses don’t have a problem differentiating the two. Their understanding of timing and intent is really amazing.
@HeartsDesireEventing I do think it’s helpful for demystifying the half pass for people who have never ridden it or just introducing it to the horse. But I don’t school the half pass by just riding haunches in on the diagonal, if that makes sense.