, when I first genuinely started riding (at 50), my mare was a spook monster. Most of the advice I got on riding, from local instructors or on the Net, was to keep shoulder / hip / heel in a vertical line. But that so obviously was NOT working when Mia would react. It was a cutting video that got me to try combining the advice from Littauer on what most people consider a jumping position and what cutters did when their horses crouched or spun. After all, Mia's spooks looked a lot like a good cutting horse moving...and it worked.
That was when I started to realize that there is a dressage seat (central balance), a forward seat (forward balance) and a defensive seat (weight well behind the stirrups). Then I finally started to understand what Littauer wrote about riding being balance in motion. I eventually concluded that all three approaches to riding had value and now find myself using all three during my daily rides. Often all three within a 5 minute period.
"At first when learning how to ride you must think about your position all the time, and in this period of your learning your picture matters a great deal. But later, when the contour of your position is correct when your spring, grip, balance, etc. are working effectively then there are only two criteria of your position; a) are you in fluid balance and rhythm with your horse or not? b) does your seat enable you to control your horse efficiently?" - Common Sense Horsemanship by VS Littauer
FWIW, I think all horses should eventually learn BOTH to spend lots of time on a loose rein AND to accept contact. Like riding position, there are times and situations where both are as useful to the horse as to the rider. Bandit had been raced using a bosal with a bit as an emergency brake. It was quite a surprise when I first trotted him and then asked him to slow using the bit. He slammed on the brakes like his life depended on it. He also raised his head high and my face got buried in his mane! So one of the first things we worked on was accepting the bit.
Not in the sense a lot of people use it when talking about teaching a horse to "seek contact". I disagree with how they go about it. But he needed to understand that I could take the slack out without causing him pain, and then learn I could use the bit to ASK him for things. Eventually that I could use the bit to ADVISE him on options that we were discussing.
Seems to me good riding includes both loose reins and contact. Like the various positions, I alternate back and forth on every ride. I'd hate to be limited to either / or.