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. See Half Pass - Correcting Problems 1 - Horse twists the head and Half pass - correcting problems 2 - changes to rhythm for more information on riding half pass..
Unlike shoulder-in, the horse is moving towards the bend, i.e. a left half-pass has left bend. It is of great value to the suppleness of a horse and the engagement of the hind legs.
A correct half pass will flow smoothly from one leg to the other, with the horse maintaining the bend round the rider's inside leg and the connection with the outside rein
The horse's quarters lead
In half pass, the horse's forehand should always be slightly in front of the quarters, This can be a difficult balance to reach, as it is not uncommon for a horse to lead with the quarters on one rein (the easier side) and trail with the quarters on the other rein (the stiffer side).
If the quarters are leading, move your outside hand delicately towards the horse's neck in the direction of movement. This will bring the horse’s shoulders across more and put them in front of the quarters.. If you make an abrupt movement with your hands you will unbalance your horse.
Ensure that your outside leg is not over-acting. Often the outside leg is only required to start the movement and may not be needed every stride. Touch the leg only when it is needed.
Horse's shoulders lead too much
This can be caused by the outside rein acting too often or too strongly, an imbalance of the rein and leg aids, or a loss of impulsion. To correct this, resolve the initial cause, and then, always, put the horse’s shoulders back in front of the quarters, don't try to move the quarters over more. Therefore, with the shoulders leading, move the outside hand away from the horse's neck in a light, delicate action. A big movement of the hands will disrupt the horse's balance.
Diane Followell Classical Dressage Trainer