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. (for more information see Half Pass - Correcting Problems 1)
Changes in rhythm are quite common and are due to a variety of reasons. It can be difficult to understand why the rhythm has changed, but once you can identify why the rhythm has changed, the correction is straight forward.
Rider blocks with rein
This is a very common fault, when the rider uses the inside rein to create or maintain the half pass, rather than the legs. The outside leg asks for the side step, and the inside leg gathers the horse, maintaining the impulsion and the bend. If the rider doesn't use their legs in time with the horse's rhythm, or uses the legs together, the horse starts to rush, or is blocked, and the rhythm changes.
The horse blocks with shoulder
If this occurs, the horse presses the inside shoulder into the half pass. The correction for this is by using the inside rein away from the horse's neck. By opening the inside rein, the horse is encouraged to stay in the correct bend and is lead into the half pass. It is vitally important not to pull the rein back as you do this or the horse will be blocked in the hind leg.
Horse quickens towards end of half pass
Sometimes a horse will speed up as they feel the wall approaching, to correct this, first decrease the pressure from your outside leg, and stop the half pass 2 or 3 metres from the wall by tactfully riding forwards.
The horse changes the rhythm
In this instance, the horse performs a good half pass but the rhythm is not consistent throughout. Ensure that your horse is bent round your inside leg and lightly connected in the outside rein. Once in half pass, check the use of your legs; using them out of rhythm with your horse will quicken his steps and alter his natural rhythm. Each horse has their own rhythm and it is important that you ride him in that rhythm.
For some horses, if the angle is too steep, they will lose the engagement of the inside hind leg and the rhythm will alter. This may be due to conformation, or suppleness, so ride the half pass at a slightly smaller angle.
Alternatively, it may be that you are asking too many steps and your horse is not able to keep the movement for so long. In which case, ask fewer steps, and gradually increase the number of steps over a few days.
Diane Followell Classical Dressage Trainer