Straightening - Quote from Alois Podhajsky:
"In order to spare himself the effort of stepping under his body with the hind legs, the horse becomes crooked, that is, he swings his hind quarters away from the track and the hind feet no longer step into the hoofprints of the front feet but to the side. During the entire training the instructor must call the pupil's attention to this severe fault.
The horse is straightened when in motion. It is easiest for the pupil to straighten his horse at the trot along the wall. The rider takes the forehand away from the wall with both reins until the hind legs follow the hoofprints of the front legs. Both legs push the horse forward.
The action of the outside rein is most important because using the inside rein alone would induce the horse to bend his neck to the inside but not take his forehand in."