Riding horses is like pasta, same ingredients, but different shapes. Push here and you get elbow macaroni. Pull there and you get spaghetti. It all starts with flour, or in this case, pressure. Mold your horse into penne, push him into bow ties, pull him into fettuccine.
One of the first things I learned about riding is that horses move away from pressure. A good example would be asking your horse to turn left. So I’m riding along, doing a twenty meter circle to the right. I’m about to cross the middle of the arena, but hey, I want to go the other way! I need my horse to move away from pressure, so I use my outside leg to push him to the new direction, and my inside rein to pull him there. Pulling, one might say isn’t moving away from pressure, but it really is. The action of pulling makes the horse want to avoid pressure by moving inward to relieve it.
I feel like everything in riding is moving away from pressure. Hands, legs, seat; they can push, pull, and stop the horse. Then there is pushing the gas and the brake at the same time, the half halt, which comes in handy to prepare the horse for some work that is coming up. I can use the half halt just before a change of lead through trot. The horse needs to be pulled back with the reins to fall into trot, and pushed with the outside leg to the new lead.
In shoulder in, I need to shift my weight to the inside and push with my outside leg. One could pull on the inside rein a little to pull the horse to the inside, but it is ultimately the outside rein that controls the angle. Half pass is the same, just a different shape. I pull on the inside rein, and push with my outside leg. There is a lot more pressure variations at work there as well, like shifting my weight, and using my seat.
Just like pasta dishes, there are many different ingredients in dressage. The recipes start off easy enough for a beginner, while the difficult ones require more seasonings and multitasking. It takes time to get from buttered noodles to chicken parmesan, but with practice, you will get there!