Um, I thought there were was more than one type of hackamore and that they are quite different in severity?
Well, there are hackamores -- and there are hackamores. And the two are about as different as night and day.
Let's simplify: a hackamore is essentially a halter equiped with some means of directing the horse without the use of a bit. The main differences between a hackamore and a bit and bridle are that a hackamore has something that goes over the nose and doesn't have anything that goes in the mouth.
The word "hackamore" is an Anglicized version of the Spanish word "jaquima" -- which, in turn, was derived from the Moorish word al-hakma -- and refers to a device that works off the nose, chin and poll. I don't think it's any accident that "jaquima" is so similar to the Spanish words "jaque" (to threaten or hold in check) and "jaqueca" (headache).
There are two basic types of hackamore: mechanical and non-mechanical. To my way of thinking, mechanical hackamores are generally rather harsh -- usually capable of inflicting severe pain and even permanent damage on the horse if not used correctly. Non-mechanical hackamores, on the other hand, tend to be fairly gentle -- often operating by simply allowing the rider to swing the horse's head to one side or the other. To illustrate, here are some examples of both types of hackamores....
Mechanical Hackamores Mechanical Hackamore
Used with a curb chain (not shown). The long shanks act as a lever, causing the nose piece and curb chain to exert pressure on the nose and jaw. A hard pull on the reins can break the horse's jaw. Stop-N-Turn hackamore
An extremely severe hackamore! Should only be used by an experienced rider with very gentle hands. Even a slight twitch of the reins can produce painful pressure on the jaw and nose. Little "S" Horse
by Reinsman Inc.
The least harsh of the mechanical hackamores on this page. The rope nose piece and chain curb mean a bit more "give," but if the curb is tight, a hard pull on the reins could still injure the jaw. Bosal
A fairly gentle control; works by swinging the head to the side, and putting pressure on the nose when pulling back on the reins. Reins attach above the knot under the chin. Side Pull halter
Arguably the most gentle hackamore on this page. Works like a regular halter, allowing the rider to swing the head to the side without pressure on the mouth, nose or jaw.non mechanical^^^