I thought you always sat the trot in dressage.
English is my second discipline, but the following paragraph pretty much states what I was taught, far as diagonals. It is more on where that inside leg is, on that circle, which of course, means in co relationship with outside shoulder ;
in CONFIDENCE· RIDER AIDS· RIDING· SEAT AIDS· TROTTING
The Correct Diagonal In Trot
There you are, happily trotting around the arena, delighted at the fact that you can ‘post’ correctly; until someone goes and bursts your bubble by pointing out that you are on the wrong diagonal… “Sit for two” they shout your way… And you’re thinking ‘Come again?’!
Being on the correct diagonal is to do with rising or posting to the trot. We know that when our horse trots, his legs move in diagonal pairs, the front inside with the back outside and the front outside with the back inside. This way of travelling, in the diagonal pairs, is what creates the two-time beat that we experience in trot. The 1,2,1,2,1,2… or, when we rise or post to the trot, the up, down, up, down, up, down.
Finding the Correct Diagonal in Trot
Posting or rising on the correct diagonal in trot is when you are sitting or down when your horse’s outside shoulder (and inside hind leg) are on the ground. This means that you are rising or posting when the same outside shoulder and inside hind leg and moving forward.
There is a well-known ‘rhyme’ to help riders remember which diagonal pair is the correct one to post that goes ‘Rise and fall to the leg by the wall’. This pertains to the front leg closest to the wall, or the outside shoulder, which when we are first being introduced to a diagonal, is the easiest way to establish if you are correct or incorrect.
However, the reason we ride on one diagonal or the other has actually little to do with the outside front leg, or shoulder, and more to do with the inside hind leg, the other half of the diagonal pair to the outside front leg, which is creating the energy and impulsion in the trot.
As the inside hind leg comes onto the ground, your horse is at the point where he is most balanced. Your being ‘down’ at this point will help to maintain this balance, particularly around turns, bends and circles. The other consideration or factor with riding the correct diagonal is that you are equally conditioning or working your horse’s back and hind end."
In other words, your posting, far as siting down, when outside shoulder is down, co responds to the fact that the inside hind leg is down at that point also,, which is the second half of that diagonal pair, and the important part, far as whether you yourself are up or down at that point