An overcheck has a practical purpose; if a horse lowers its head too far when in harness, particularly if moving quickly, it can catch the bridle or reins on the shafts of the carriage or wagon, risking an accident. Properly adjusted and when used for a limited period of time, an overcheck does not significantly impede the motion of the horse or cause discomfort. If too tight, however, an overcheck rein can be uncomfortable because it puts strain on the neck muscles and ligaments. On a hard-working draft horse, it generally is not used at all, as it puts the animal off balance by preventing it from lowering its head when trying to pull significant weight. A horse pulling weight normally lowers and extends its neck in order to push against the harness. If an overcheck is inappropriately used, or used for too long a period, spinal strain resulting in neck or back injury can occur.

https://en.m.wikipedia.org/wiki/Overcheck