Sentence:  Remember, when a horse is on its side the kicking-strap is useless, and the <WSD>breechen</WSD> nearly so, to prevent the swing of its hind legs; your aim is to get it away from the steps or wheel-plate of the trap, which, in its struggles, may cut its legs fearfully. 
Meaning of the word marked with <WSD> tags:  The part of the harness that fits over the horse's rump and holds the load back or permits the horse to back it up [http://www.gaitedhorses.net/Articles/HorseGlossary.html]