We seem to have a mass fixation at the moment in the horse world with what the horse’s head and neck are doing. As long as the neck is arched and the face is vertical or behind the vertical (which is not good for any extended period of time, however many photos we may see of ‘top’ horses in this position) a multitude of sins elsewhere (not tracking up, not engaged, hollow braced back) are often ignored.
A horse working properly will have a soft, swinging, rounded back, and the feel you are looking for is very similar to when a horse has a poo under saddle. The back rounds upwards under your seat. I worked this out when a former dressage trainer of mine was riding one of my horses, who was being a bit recalcitrant that day. After quite a bit of work, Orville (the horse) starting doing a poo (at trot), at which point my trainer said “Ah, that’s more like it” and then realised it wasn’t quite what he was looking for yet!
Nevertheless, it was a lightbulb moment for me. Ah, THAT’S the sort of feeling you are aiming for! Wow.
I liken it to sitting on top of a mushroom, I find that a really useful image. It is slightly spongy and bouncy and rounded, the way a huge mushroom would feel underneath your bum!
Of course the hind legs should be swinging through, the picture should look balanced and, ideally uphill (depending on the individual horse’s conformation, age, and stage of training) but the most tell-tale sign is that the area behind the saddle should be soft and up (or, at least, flat) and not dipped/braced/tight.