The shanks on the mechanical hackamore provide leverage, just as the shanks on a curb bit do. Rather than pressure being applied inside of the mouth, the hackamore places pressure over the nose. When the reins are pulled, the crown of the bridle is pulled down against the horse's poll, the nose piece is pulled against the horse's nose and the chin strap applies pressure against the chin.
The severity of the hackamore depends on the materials it is made from and the length of the shanks. The longer the shanks, the more leverage can be applied to the horse's head and face. A rounded nose piece made of stiff lariat rope or braided leather around stiff cord will place sharper pressure against the horse's nose piece than will a flat wide nose piece. A chain under the chin will be a sharper aid than a leather chin strap. Pulling back on the reins will bring the nose piece and chin strap closer together, pincering the jaw.
Hackamores are not more humane because there is no bit in the horse's mouth. In fact, hackamores can be very harsh, causing severe pain to the horse's sensitive face. The shanks on some hackamores can be over 8 inches long (20cm). With the force of leverage it is possible to damage a horse's face. It is important to adjust the nose piece of a hackamore high enough that it sits above the cartilage of the nose.
A mechanical hackamore is not recommended for training a horse. The action is too severe, inaccurate and the horse must understand how to neck rein and halt on a loose rein with a light touch. It is very difficult to teach a horse to bend or flex because it is not possible to direct rein with a mechanical hackamore. Nor is it a good idea for a new rider with unsteady hands to ride with a mechanical hackamore.
The main benefit of using a mechanical hackamore is that through leverage, a hard to stop horse can be stopped with minimal contact on the reins.. Although these aids might look appealing as a quick answer to teaching a horse to 'whoa', they can also cause problems like head tossing, high head carriage or over bending, and potential facial damage. Trail riders looking for a bridle that allows the horse to eat and drink on trail might consider using a bitless bridle, bosal or side pull . That way the horse is less likely to inadvertently hurt itself by hooking or bumping a shank.