As often happens, as I was thinking and writing and focusing on the phrase "on the bit" the past two days, an email from a list I'm on popped through and mirrored some of what I was trying to express.
I'm paraphrasing, but this is the gist:
In conversation with one of the Spanish Riding School instructors, the instructor offered that the rider should never ever close the seat - but in fact should do the opposite, especially in transitions.
He noted that the SRS asks that their riders be capable of allowing the horses to come "through" utilizing 6 points of contact - calves, seatbones, and hands.
The scales of training cannot be attempted unless the rider is absolutely relaxed with a wide seat at all times - which is why it takes so long to develop into a classical rider and why time on the lunge is so precious.
If a rider tightens the seat at any time, the horse will tighten his back in response, which will create discomfort and put the horse onto the forehand and/or increase the front leg action.
He notes that with horses even a tiny bit of tension is felt as a lot of tension.
The above is why I think the phrase "on the bit" can be so dangerous in its broad, misunderstood usage.
Six points of contact must be independent but at the same time utilized in harmony in order to create a horse who is "through" and on the aids.
But nothing can proceed classically without the complete relaxation of the rider, most importantly with the rider's seat. After all, this is where the weight of the rider literally bears down on the horse, onto a particularly vulnerable part of his body.
For riders not capable of relaxing their bodies completely (how many of us can even come close on a daily basis?) the horse is continually protecting his back against our tension by creating his own tension, which then brings everything we're trying to accomplish to a halt.
The focus of getting a horse "on the bit" - as it is most often used - misplaces our attention totally.
Even "on the aids" is confusing, as so many riders don't even consider the seat an aid. Think of instructions you hear from the vast majority of riding instructors: glue yourself to the saddle, plug into the saddle, etc. I see riders pumping the canter, grinding the sitting trot and the canter, posting like jack-in-the-boxes, many times while legs are pinging with every stride, and forearms are stiff. In worst case scenarios, the reins and contact to the horse's mouth has become the balance bar the rider must hang onto to accomplish all of the above contortions.
We should all be hearing instructions that focus on the relaxation of our bodies - I was fortunate that when I came back to riding I did it with a classically trained friend whose instructions to me as I went around on the lunge line week after week were: breathe, breathe out, look over your outside shoulder, let your legs drape quietly, breathe, breathe, breathe, close your eyes, feel your seat bones, etc.
The answer to every single problem I encountered in the saddle was to breathe.
She didn't allow me off the lunge line until I could use my seat (and breath) to initiate a walk, trot, change of tempo, and halt with my seat alone. And if she saw me "scooching" at all while doing it, it didn't count!
She is the trainer who taught me to "think" half halt - as opposed to doing anything with those six points of contact. "Think" the transition. There is always the opportunity to consciously add aids, but if you "think" them first, you at least allow for the subtle energy aids that our horses understand and respond to so easily - if we only let them.
Really, if we were smart, we'd teach every new rider to "get on the relaxation" with nothing else said until they do that as a matter of course. Children are good at it - and if we make the connection for them early, by showing them that the horse or pony responds when they breathe out, when they drop their legs, when they close their eyes and feel their seat bones move, they can carry that with them as the mantra for creating beauty and harmony in their riding, and in their relationships, with horses.
So, I fell up onto a soapbox here, but when it comes to being "on the bit" or being "on the relaxation," I think this is actually not a soapbox, but higher ground.