Circles, Corners, and Other Curved Lines
One of those expressions that most of us have heard but seldom practice is that dressage riders "get" their horses in the corners. Actually, most corners, circles, and other curved lines are seldom used to their best advantage. When executed properly, these figures aid not only suppleness, but also balance, straightness, impulsion and collection. However, most of these benefits are usually not realized because of poor execution.
In general, most riders tend to give on the outside rein and take on the inside rein when performing any movement involving more bend. This may be an involuntary and unconscious action on the part of the rider, but it has a detrimental effect just the same. It results in a slightly lengthened rather than slightly shortened frame. The horse ends up more "strung out", because the bend is more in the neck than around the inside leg; therefore, the outside of the bend gets longer rather than the inside of the bend getting shorter.
If, on the other hand, the rider initiates the turn with the inside leg and "catches" the resulting increase in impulsion with the outside rein while opening the inside rein to indicate direction, the horse will respond by bending around the inside leg and shifting the balance back. Then the rider can give on the inside rein and drive, rather than pull, the horse around the turn, thereby completing the half-halt. Of course, the rider's outside leg must be slightly back in order to guard the haunches. If the horse's hind leg steps out, the horse will have again successfully avoided bending in the body.
This should give the rider some insight into correct bend. The bonus is that it also helps to understand half-halts, how giving in the poll is a function of bend, and riding the horse from back to front - not the other way around.
These are obviously not minimal accomplishments (!) and explain why dressage riders are endlessly performing circles, corners and other curved lines.