Are you a person who learns by watching, by feeling, or by researching? I ask because it makes a difference how to explain it to you. I have spent years riding dressage up to 4th level, and I never REALLY understood how to sit the trot until I red Syliva Loch’s book The Classical Seat, and Riding in Balance.
For YEARS I was told to sit up, sit back, lean back on my seatbones, make my back a D, etc. NONE of this worked for me. In fact, most of this was absolutely the opposite of what I needed to do. Now, maybe if you are a hunter rider and lean WAY forward on your crotch, then yes, you need to sit up. But basically, leaning back is the easiest way to bounce on your seatbones.
In a nutshell, to feel the triangle of support you must sit on, sit on your horse at a halt and pull your legs in FRONT of the knee flaps. You will be aware of your seatbones protruding down and your pelvic arch. THAT is the base of support for all dressage work.
Now, put your legs back down and feel your long thigh bones against the flaps of the saddle. If you have big thigh muscles, or “fluff”, pull this back behind your leg so you can feel the saddle right against the thigh bone. Those are your stablizers side to side.
The lower part of your leg hangs gently at the side and is available for leg aids.
Above the triangular base of support, your back (meaning spine) needs to be in a slight “S” shape in order to have the ability to absorb the movement. There needs to be a slight arch in your lower back, stretch tall, feel the pelvic arch in contact with the area behind the pommel. The shoulders are up and back, but everything is just vertically plumb, NOT leaning back at all. Upper-level riders occasionally lean back or LOOK like they are leaning back because of the relative lift of the shoulders, but don’t START that way.
Now, when you trot, your horse HAS to have his back up and loose for you to sit on it. If he is hollow, it is not worth trying, and you will only exacerbate the problem. If he is in front of your leg, on the bit and supple, then lower your triangle area onto the saddle and think of a “back-pedaling” motion with your hips. That is the closest motion I have found to the real feeling. Do not try to be still, try to pull the back up with your thigh bones and then push it down with your triangle.
Riding without stirrups helps, but only if you are not terrified. Otherwise your legs act like a clothespin and pop you out of the saddle by gripping so hard.
I have a large-ish butt and have to actually think of having my butt stick out behind me like a duck. It doesn’t LOOK that way, but that’s the way it feels if I’m in the right position.
As far as the “D” shape goes, I think the shape is actually backwards. I think your back is the straight part of the D, and your belly is the rounded, soft part. I’ve been told for so many years to suck in my gut, it was hard for me to relax and feel my abs absorb the motion. You just can’t do it easily when you are tensing your abs and psoas. (I think that is what they are called.)
Anyway, as you can tell, I’m really into the “seat” thing right now, but it has really transformed my riding. It is worth the effort to figure it all out.
Good luck!