The outside rein connection is integral to dressage. Riders should strive for the outside rein to remain consistently and elastically connected throughout their work. Why is it so important? It's what allows circles, corners, and lateral work to improve the balance and self carriage of the horse.
When working with a horse that does not have a great natural lengthening, make sure that you don't ask for too much too soon. Compare this to a singer who is training their vocal cords… they gradually expand their vocal range until they hit their limit. This helps
A good halt is square because it was a good halt. Not because the rider has taught it the "trick" of squaring up after an unbalanced halt.
If your horse finds lateral exercises more difficult in one direction vs the other (as many do), then they are not truly straight, or evenly laterally supple.
"I used to ride mostly Thoroughbreds. I don’t know if they’re more intelligent, but I find that they think for themselves a bit more than the warmbloods. With a warmblood I find I definitely have to do a lot more, and think a lot more for them.