If the rider's aids are precise and well-timed(QUICK), there is no need for strength. That is the lightness we're seeking: a precise system of communication that develops a clear understanding. Lightness and harmony depend on precision and timing.
This is, of course, the ideal. As was pointed out to me by my coach, Rachel Saavedra, recently, Dressage is a sport of "ideal". When you read the books, watch the video tutorials, and hear the interviews of the top riders, you are usually only hearing about the brilliant moments...the highlight reels. It's the tip of the ice berg. As David Wightman once told me, there's a whole lot of ugly before it gets pretty.
If you were taking gymnastics, or dance, for example, instead of a riding lesson, and your coach was teaching you a new movement, your coach would first break the entire sequence down into the smallest sized pieces, and then further simplify by SLOWING it all down. Once you learn each piece of the movement correctly, with good biomechanics and a clear understanding of the desired outcome, only then would your coach have you speed up the sequences and meld them together to form an entire movement. This would happen for every part of every dance or gymnastics routine. Same for martial arts and other endeavors that involve movement.
It is much more difficult to do this when a 1200 lb animal is involved in the educational process. The horse doesn't speak your language; he is extremely sensitive; and the vast majority of riding coaches have only been taught to ride, and not necessarily taught to TEACH.
I'll give you two exercises to help you improve your timing and precision. These can be done in the walk.
1. Post to the walk. Go large around the whole arena. Let the horse have an active, ground covering walk, and do not spend much time futzing with the reins. Let him have room to move out. If you have an enclosed arena, go on the rail and eliminate as much steering noise from the conversation as possible.
This is MUCH more difficult that posting in the trot for a couple of reasons. First, when you post in the trot, you have the benefit of the thrust from the horse's hind leg which will literally push you out of the saddle. Most people learn to post the trot by letting this happen, and seldom go beyond this phase to learn how to dictate how the horse uses his hind legs and back with their own posting mechanics. Secondly, it's more difficult to post to the walk precisely because it is slower, so, you have to have sufficient tone to hold yourself up long enough to stay on the correct diagonal and in the correct timing, vs. in the trot where you can just push off your stirrups.
In order for this exercise to be effective, you must PICK the diagonal you are rising to(and be sure to CHANGE the diagonal frequently), and you must STAY on that diagonal until you decide to change to the other diagonal. One of the keys to being precise in the application of your aids is rider tone, strength, and symmetry. We all know we want it from our horses, but how many riders spend at least as much time each day demanding it of ourselves as we do demanding it of our horses? Posting the walk will strengthen the muscles of your lower back, abdominal muscles, and upper legs. It will improve your balance as well. And by staying on the diagonal you've chosen, it will improve your eye as well as your feel over time.
2. Cue the horse's hind leg just as it is starting to leave its stance phase and transition to the swing phase. Again in the walk, going large, with the same directives as above, learn to feel when the horse's inside hind leg is beginning to swing. Be sure to change the rein frequently so that you can work equally on learning the timing for both hind legs. It will either BE different, or, FEEL different, due to horse and rider asymmetries, strengths, and weaknesses.
Every leg, in every gait, has a stance moment, and a swing moment. If you have a ground person, they can help you by calling out "now" every time this happens with the inside hind leg, so you can listen to the timing as you ride. If you have a mirror, you can watch in the mirror. If you have neither, you can literally look down to the inside hind leg for a few repetitions, until you can relate what you are seeing to what you are feeling.
Once you have the timing down in your mind, pulse or tighten your own inside leg at the same time. It can be a little bump from the entire calf to heel; it can be just the outer hip muscles; or it can be the whole leg, hip to heel. Try them all. See what works. Make the engagement of your muscles be in the same tempo. In other words, do not just keep the muscles engaged. This is extremely important. The horse learns from the release.
Remember I said above that horses are extremely sensitive? How many readers immediately thought to themselves..."My horse isn't sensitive; in fact, he's the laziest horse on the planet!" I assure you, it is the dull/lazy horses that usually are the MOST sensitive, and frequently also the smartest. Becoming dull and/or lazy is a coping mechanism they've employed to drown out all the conflicting messages they are getting from their riders. Here's your moment to look in the mirror and realize you've contributed to this. Now, get over it, move on and commit to learning to be more precise. It won't be easy, It'll take time, effort and focus. But, if you stay the course, eventually even the dullest horse will become re-sensitized. I promise.
As you get better at your timing, try to notice what part of the aid your horse is responding best to. You will likely find that as you get more and more precise, you can use less and less aid to get the same amount of response. I find that the horses who are most resistant to the leg are usually the most sensitive, and would probably respond best to just your hip. You may have to explain yourself with a very precise touch(and I DO MEAN TOUCH, not strike) of your whip at the same time as you engage your leg. Make the touch of the whip as light as a fly landing on their side. The good news is that in the walk, there is no impulsion to jostle you, so, you can take your time to learn to be very precise.
Of course, eventually you'll want to trot and canter, because the horse needs to. But, spend five or ten minutes of each warm up and cool down phase playing with these two exercises, each day. And of course, there are always caveats to every exercise. As Janet Foy says, everything we teach our horses will be used against us later! For example, at some point, you'll need to learn to time your aids to emphasize the stance phase of the hind leg or your horse will never learn to keep his motor running when you try to initiate a downward transition. A topic for another day!
Namaste.