I have to somewhat disagree.
Seat IS important yes, but for me, leg comes first, which drives the horse forwards in to the hands, making for a comfortable seat.
I funnily enough had this conversation with Ingrid, my trainer last night. The worst thing she sees is people rubbing around in the seat, over using leg aids and pinning a horse's nose to its chest.
She said the legs are extremely important, once you crack that, you crack the seat. If you pinch with your knees- you can't sit etc, things like that.
I see your point and it's alright to disagree :P And yes.. that kind of scenario of not riding properly and forcing your horse into some kind of insane shape.. I'm not a fan of.
But I was more talking about the seat bones, shifting weight, but a good seat in general, an active (correct of course.. which takes time and muscle building) seat is what I consider the foundation to good riding.
last night was the first time I managed to do sitting trot on my greenie and FEEL every step, ride figures correctly and do some pretty fab transition work. Quieter, deeper more relaxed seat, not niggling, wiggling and tensing up as I have been doing.
I'm afraid Nuno in his excellence and skill far outways any of us including you on this forum.
I'm not a professional, but I am entitled to my opinion. And I do highly respect those riders and whatnot, but all I know is what I feel.. and the difference between a framed horse and an engaged horse.
Take today for example.. asking for a walk --> trot, I use my seat and then add the cue of leg. When we picked up the canter from the trot (on the lunge of course since I'm still learning myself) I noticed it was a lot smoother and a lot less jagged of a canter when I was using a driving seat and I felt the rhytm. I didn't even use my legs to keep him going.. it was just all seat. When you pull the horse back down to a trot.. the first thing you do is get lighter in your seat. You sit up and then sink down into the transition (along with half halts.) You keep your leg on but what makes the change.. is the seat.
Hence why I am so strongly for the idea of it being the most important thing. Sure you could teach a horse to work off of hand.. but they probably wouldn't be connected :/