The ability to move the horse’s head left or right without resistance is one of the corner stones of the training process. Without lateral flexion it's impossible to ride correct circles and corners, lateral movements or even just a change of direction. In fact a lack of flexion will affect everything you do, or attempt to do with your horse! There is an old saying that ‘...a flexible body is a flexible mind’. Certainly, the more control you have of your horse’s head position the more you can influence his state of mind. Even in a trail riding situation the ability to flex your horse laterally is a key tool. If your horse has ever started to get a bit tense - for example when other horses appear on the horizon (even the local hunt) - by keeping the neck soft you can influence his state of mind and re-establish a sense of security - for both of you!
Imagine a young horse warming up at a show under saddle for the first time. It's new territory - literally - and they're not going to behave exactly as they do at home where everything is familiar and they feel much more secure. Some will be more reactive than others, but you can guarantee that they'll be looking around, distracted by everything and probably having a spook or two! You need to be able to do something about it. The good news is that if you're able to draw his attention back onto you by flexing him laterally he'll be unable to stare at all the distractions, and he'll be less likely to spook! You'll also increase his confidence and his trust in you, as he is now relying totally on your directions.
The importance of feel and timing
Teaching a horse to flex will test your feel and timing. It's impossible to achieve a responsive, soft horse without the rider/handler developing their own skills. It's vital to fully understand pressure and release in training. (See "Pressure and Release" in the Resources section). Working on your horse’s flexion at a stand still, both from the ground and under saddle, is an excellent way of improving your ability to communicate to your horse.
Finally, when you have your horse flexing well left and right (ie equally soft and supple in both directions) use it regularly standing still and whilst walking, jogging and loping.