1) The rider's leg is activated by squeezing the bum muscles on that side
I was watching a Dressage Naturally video and Karen mentioned squeezing her bum muscles to move the horse sideways. I had never ever heard this before. Went out and tried it. Bam a horse that went sideways MUCH easier. In fact with very little leg, just the amount of leg pressure generated by squeezing the bum muscles on that side. Try it sitting in your chair and feel how much it activates your leg, then try it riding. You'll thank me. Add in the slight weight in the direction of travel below and you will thank me again.
2) Leg yields and lateral work are less about leg/rein aids and more about rider positioning and weight shift in the direction of travel
Again, I was watching a DN video about lateral work and all the various aids that can be used. There were lots of great things in the video but weight in the direction of travel was an eye opener for me. We all know that the rider's energy is pushing forward from the center going straight or on a curved line. Like a line of arrow coming out of the rider's center toward the desired destination. What happens though, when leg yielding, in lateral work, or various other sideways movements? Well, in leg yield the rider's weight shifts slightly sideways in the direction of travel to tell the horse which way to travel. It is very subtle, think rib cage slightly towards elbow, not leaning tower of Pisa. Add in the squeezing of the bum muscles to activate the sideways leg and really good things start to happen. In the lateral work the direction of travel is on a diagonal so the rider's weight shifts slightly that way. For instance, shoulder-in right the energy would be going right to left so the rider's left rib cage would be slightly pushed toward the left elbow. Haunches-in right the energy is going left to right so the rib cage shift is right to right elbow. Opposite obviously, for lateral work left. Sit in your chair and do your normal aids for lateral work and then try these subtle weight shifts while thinking of the arrows. Your horse will thank you.
3) The rider's spine must have a gentle curve to have a truly following seat
I was watching a free video from The Online Riding School on Facebook about rising trot and keeping a gentle curve in the rider's lower spine on the landing phase. My back has been bothering me slightly and I wondered if maybe my lower back was too flat. Well, I went out and tried it. No back pain, much freer and forward rising trot with no effort. Then I tried it at the sitting trot and canter. And again, no back pain, easier to follow the much bigger trot and canter that Jet automatically offered. By riding with my lower back just a little too flat my hips couldn't follow as well AND I was essentially riding with the parking brake partially on.
4) Conversely, the rider's spine flattens that gentle curve to slow or stop
So, if the curve in the rider's lower back allows the rider's hips to follow the horse's back and sit deeply what happens when the curve is taken out? Sit in your chair and try it. Sit in riding position with a slight curve in the lower back, allow your hips to follow a walk or trot and then take the curve out of your back. You stop following so your horse would probably slow or stop. Some people describe this as thinking of a string pulling your pubic bone towards the top of your rib cage in a "U" shape. Other people describe this a squeezing your seatbones or knees towards each other. Either way, the rider's back flattens slightly, the seat stops following as much, and the horse will naturally slow or stop. Try it with no rein contact and see what happens.
5) Breathing in as the "get ready" signal and breathing out to execute the aid makes things much clearer for the horse
This one I have been playing with for a while but it is so important. The breath in is the signal to "get ready" and the breath out is the "do". It amazes me how tuned horses are to breathing and how it is an aid in and of itself. Sit in riding position in your chair. Practice some basic transitions and figures but add in the breathing in to get ready and the breathing out to execute. Feel how it changes your posture and the clarity of your aids? It only takes a couple of rides for your horse to pick this up as a cue and it is a game changer for more subtle aids.
Looking over this post I'm struck by how all of these things fix problems that in conventional riding instruction would commonly be solved with rein or leg aids. Instead, these ideas use the rider's body, breathing, and core and lead to a much more subtle level of communication between horse and rider. Probably all things "talented" riders do automatically or pick up quickly and the rest of us have to learn the hard way, step by step.