The half pass is a lateral movement where the horse takes steps to the side and forward, moving in a diagonal line away from the starting point. Unlike shoulder-in, the horse is moving towards the bend, i.e. a left half pass has left bend. It is of great benefit to the suppleness of a horse and the engagement of the hind legs.
Half Pass essentials from the FEI rule book;
To ride half pass, the horse must be placed round the rider's inside leg and lightly connected to the outside rein. Evidence of a good half pass is arriving at the end of the movement with your horse still connecting from your inside leg to your outside rein. Many problems occur when the rider creates a half pass by using the inside rein to force the bend and the outside leg to push the horse in the direction, abandoning the inside leg and outside rein.
When the horse is able to do shoulder-in from the long side on to the centre line, you can begin some steps of half pass. Start with your horse in a shoulder fore or shoulder-in position. The degree of the angle depends very much on each the horse; for novice horses, have a shallow angle. The inside leg gives the bend, and then the outside leg, slightly behind the girth, pushes the horse to the side.
It is important for the inside rein to yield and the legs to alternately touch and release, or the horse will become blocked. The rider's shoulders should be turned slightly in the direction of the movement so they remain parallel to the horse's shoulders, and the rider's seat should be balanced across both seat bones.
The horse twists his head when:-
Usually these errors need to be corrected by going back a level in the training and spend some time placing the horse correctly around circles and shoulder-in, ensuring suppleness on both reins.
Then set up the half pass again, ensuring that you don't take the inside rein, as this prevents the inside hind leg from coming through and creates resistance on the inside rein, blocking the horse.
If the horse is stiffer on one side, return to shoulder-in to supple them more before starting half pass again.
Initially, you may need to ride half pass at a slightly shallower angle until your horse is comfortable in the movement, then you can gradually increase the angle and the number of steps.
Don't use your legs together as this will confuse your horse. If necessary, touch lightly with alternate legs, in rhythm with the movement of the horse.
Diane Followell Classical Dressage Trainer