Well thankfully Di appeased me with a little debate. If I'm reading your comments correctly Di you are talking of the spiral seat where the upper body turns independantly from the hips and lower half, in other words a twist in opposite directions at the waist?
This is something I would use in later work, with my horse that can walk/trot/canter on circles and straight lines. I think it's a great tool to be able to ride the horse into the outside rein with the seat but keep him bending around the inside leg with the upper body. However, when I've ridden it I've felt that it is a momentary aid whereas the seat and upper body turning in unison is a staple initial *layer*.
When turning in synchronisation I use my outside knee and thigh to keep the outside shoulder turning and I find that if I approach the turns in a *square* manner then the 1/4's are placed correctly and it is tricky for them to evade/drift out. My shoulders turning ever so slightly onto the same arc the horse is on will bring my outside rein slightly closer to the wither and will back up my outide leg feeling of guarding the outside shoulder.
In all of this turning work I can't stress enough the importance of keeping both legs long and around the horse. Any unbalance in the rider during this early turning will be disastrous for the turn/circle and for this reason I always use my *breathing* legs deeply into my stirrups (like a half halt really) before applying the turning aids.
Of course the reality of all this is that our seats are in a constant state of flux in order to be with the movement of the horse, this goes without saying. Also that the riders body will only turn as much as the circle requires, any over turning by the rider would only be used as a small corrective movement.