Many riders know that this is an exercise that is taught, but the reason behind this exercise often is unknown. Turns from a halt can be either around the forehand or around the haunches. The center point of the turn is next to the horse's inside forefoot in the turn on the forehand, and next to the inside hind foot in the turn on the haunches.
The turn on the forehand is an exercise aimed at developing looseness in the horse and also teaches the horse to respond to your sideways pushing aids. As this exercise is performed slowly, this can be an excellent opportunity for the novice rider to learn and practice each aid, but most importantly to coordinate all the aids.
Initially, the turn on the forehand should be performed on the inside track, not against the wall to avoid making the horse stepping back or raising his head. Start at a halt. Your horse should be flexed to the side of the sideways-pushing leg, the direction of the turn.
The horse's inside hind foot steps in front of and across the outside hind foot. At the end of this exercise (when your horse has turned 180 degrees), he should be straight and at a halt and should be back on the outside track.
So, what aids are used in this exercise? In order to get the flexion, you must transfer more weight onto the inside seat bone and shorten the new inside rein. The outside (guarding) rein prevents excessive bend in the head and neck. With your inside leg drawn back ever so slightly, push the hindquarters, step by step, forwards and sideways around the forehand. Use your leg in time with your horse's movement.
Your outside leg should prevent your horse from stepping too far sideway and prevents this movement from being hurried. At the end of the movement, as mentioned above, your horse is halted.
Remember, that throughout the turn, you must use your legs and weight to keep the horse "in front of the leg." This is the only way to make sure that your horse will remain on the bit and will not step back or forward.
Good luck, and go practice that turn on the forehand!