I think that question would take several chapters in a training book to answer.
We need to define what kind of lateral flexion first.
There is the kind where you ask the horse, with a halter, bosal, snaffle, whatever you have, to give laterally, a hint of giving at first, then a bit more, then over time and the horse getting more flexible, where it gives without straining against it, willing and soft, you may ask for as much bend as you wish.
Doing that both ways is seen similar to stretching exercises.
You can also do the same as carrot stretches, where you lead the horse’s nose around with a carrot or other treat, the horse follows.
When you do that with the horse standing there, you are working the horse in a way that you don’t want it to work when you are riding, because you are disuniting the horse, you are separating those important balance muscles involved from the rest of the body to move them by themselves.
If you overdo that, when you ride the horse and ask for a bit of give, the horse will give, but break at the poll or any other place down to the withers and cheat on you, his giving is not translating to moving his whole body as one, ready for whatever you will ask next.
That cheating is called rubbernecking, a hard bad habit to correct and an easy bad habit to teach and reinforce for those wannabe trainers that only see the giving, don’t know any better, don’t realize the horse is not responding but by disuniting, not a good idea generally on a riding horse.
Now there is the other way of flexing to get a horse to give softly as it is receiving the energy that the front is receiving from behind, as the horse collects properly.
There is a fine line there with giving, a soft feeling all over the horse, with giving so it, again, breaks somewhere on the neck and disconnects.
That is best done while the horse is moving, where it is engaging it’s whole body.
One way to get a horse that tends to overflex giving properly and still keep the energy flowing is to use more leg and MORE LEG, yes, almost shouting with your leg and seat, something aggressive trainers like Clinton Anderson can do in their sleep, so well they never even know there may be a problem, because if a horse start cheating, they immediately just push it on.
Well, others don’t like a horse that is quite so finely tuned to giving, they want giving, just not overflexing, so they then don’t have to correct.
In a way, a bit of flexion, done properly, is fine.
Doing it to every horse, most of the time, while you are riding them, as some do, well, you just end up, easily, with a spaghetti necked horse, if you are not careful.
There is way more to this, how to have the horse, when you pick up the reins, just that motion, on a drapey rein, the horse already uses his back properly and curls up on your hand, without really flexing, but you can feel the energy flowing under you and the horse is then ready to move from behind, elevated in front.
When you have that feel, you can do anything, even flying lead changes, because the horse is so soft and willing as to be an extension of you.
You can learn all those concepts best if you get with a really good trainer and get to learn on a finely tuned horse.
Once you have the feel for that, then you can learn to look for it on any other horse you ever ride, a bit like once you learn to ride a bicycle, you can ride any other.
I have seen colts bred for that kind of athletic and mind combination already offering that kind of ride the first time on them, but any horse can learn to move like that, even if they won’t be stars at it.
I hope all that helped some, where you can go find more on your own.
The technical parts of riding and training and performance are about the concepts behind it and this topic is one of those.
