EXTRACT FROM: The Allen Illustrated Guide to Training Aids

Side reins should not be used tightly to force a horse into a certain head carriage.
If this is done the horse simply tucks its head into its chest without engaging the quarters or working the back properly and the stride becomes short and stilted. Once a horse has learnt to drop the contact with the bit it is very difficult to encourage him to take it up again as each time the rein is taken up the horse thinks it is correct to tuck its head in and drop the rein. You are always trying to achieve a state were by the horse reaches forward and seeks to take up the contact.

To fit: As a general guide to fitting side reins the reins should not be fitted onto the bit until the horse is in the schooling area in which he is to work. And then the reins should be attached to the bit so that they are straight without any tension when the horse's head is in a natural and relaxed position at the halt. If you are trying to create a very long low stretch with a deep profile then the reins can be attached side by side in between the horse's front legs looped around the girth then up to each bit ring. To use side reins attached between the horse's front legs is not suitable with a very young or over exuberant Horse as there is always risk of the horse catching the reins in the front legs. If you want to encourage a less deep response the reins attach to the girth straps under the saddle above the girth or preferably onto the side rings of a lungeing roller.