If the horse is moving its hindlegs in under the seat of the rider and the front legs in under the muzzle it is doing its basic job. It is carrying itself and the rider and you will experience softness in your riding.
When the legs of the horse are not doing its job you will notice this in different ways, depending on the horse. Some loose everything and fall all over, wobbling here and there. Some lower their back and put the muzzle up in the air, some go too low, some just loose a bit of stellning. There is a whole range of ways to loose it, ranging from minor issues to big ones. We need to figure out which leg is not working properly and then adjust it. If we allow ourselves some time and stop all negative voices in our heads about how difficult it is, we can just sit and feel our own body and the body of the horse. What is it that we sense? Most of my students do feel what is happening if they just give themselves a little bit of time and get explained what to look for.
For the horse to move properly we need to stay out of the way of the horse, i.e. allow the movement of the horse to pass from the hindlegs to the front. We need to first allow and then induce chest rotation of the horse, to get a proper bending and stellning. This means we have to sink down a bit on the inside seat bone and lift the outside. Still, you need to sit on both seat bones. If not, it is very easy to lose the outside. After all, the horse is a unity and all of it needs to be moving in the same direction. How well can you place the legs and how soft can the horse become? This is what your training should increase, little by little.
…the softer you can stay, the softer the horse will be.should
In our efforts to get the legs in one direction, it is easy to press too much with the aids, maybe adding pressure and increasing it until the horse responds. Adding a lot of pressure often means we lose our seat, and then of course it is more difficult for the horse to respond. You should try to always stay soft enough that you can maintain your seat. Use the whip as an extra aid rather than pressing too much, and if the horse does still not respond it probably either cannot do it or, most likely, doesn’t understand how to do it. Then you slow down and try again, or explain in a different way. The softer you can stay the better it is. You will be able to add your aids in a more precise way because when you are relaxed you can still feel what the horse is doing. If you press a lot then you will get stiff and you do no longer feel the horse. You will soon notice, that the softer you can stay, the softer the horse will be.