Again I refer to the Article by Bill Morong.
Camptown Harness - camptownharness.com and horseharness.com - 800-717-0957
Part 5 - The Braking System
For carriage driving, the usual braking arrangement comprises a breeching 45 and holdback straps 47. The breeching is essentially a wide strap, connected by its extension, the holdback straps, to the vehicle. When the vehicle pushes forward, force is applied to these two straps, which transmit the force to the breeching seat, which bears on the horse's buttocks. The breeching seat and holdbacks work for braking analogously to a breastcollar and traces for acceleration.
The breeching seat should be of such length that the loin strap 43 passes over the highest portion of the croup. Though they are uncommon nowadays adjustable-seat breechings can allow proper fit to different horses.
Like the breastcollar, the breeching seat is fitted (usually) with four uptugs 44, which receive the billets of a forked loinstrap. The loinstrap passes over the horse's croup, thus supporting the breeching seat. The level of this seat is adjusted by buckling into the correct holes on the loinstrap billets. The seat must press safely below the point of the buttocks, lest upon deceleration the seat ride up, causing the force of the vehicle to bear at the base of the horse's tail, likely causing a runaway turnout, kicking, or both. The seat must not be so low as to interfere with the movement of the legs. The chape of leather passing through the rings at the seat ends is subject to wear by thinning. Examine these parts carefully when harnessing up, as their failure can allow the vehicle to overrun the horse, causing an accident.
The holdback straps are looped through rings 46 at the ends of the breeching seat, through footman loops 52 on the vehicle, and around the shafts. These straps are adjustable in length by buckle holes and by wrapping around the shafts. They should be adjusted to give a clearance of four fingers between the breeching seat and the buttocks with the horse in draught. During the first few minutes of driving, the harness may stretch, necessitating readjustment of the holdback straps. The loop through the rings of the breeching seat is a common point of wear by thinning, as is the place on the holdback strap where the ring rubs. Examine these parts carefully when harnessing up.
A crupper strap 41 retains in position the loin strap, which passes through a slot in the crupper strap. The crupper strap is looped through the staple at the rear center of the saddle. It then passes directly along the spine, past the loin strap, then is split into a fork. A smooth tubular leather loop called a dock 42 is attached to the two branches of the crupper strap. The crupper dock passes under the base of the tail, retaining the crupper strap in correct position. If, as is usual, the dock is buckled on, the distance from the base of the tail to the loin strap can be adjusted. Adjust this distance before adjusting the more forward portion of the crupper strap. Then snug up the forward adjustment, but be gentle -- the strap shouldn't twang like a bowstring. The dock is stuffed with linseed (flax seeds) which are full of oil. As the dock is used, the seeds are bruised, releasing oil into the leather casing keeping it soft. It must be soft, for the underside of the tail is an exquisitely delicate and sensitive portion of the horse, very subject to being galled. When harnessing up, always examine the dock to assure that it is smooth, soft, and clean. Even a good dock sometimes disagrees with a horse if its texture, size, or way of moving is not right. This problem can cause violent kicking.
William Morong, Harness Maker