Sentence:  The <WSD>breechen</WSD> kept pressure back off the horse's hind quarters when they braked — the horses were very aware of this and paid close attention to its functioning. 
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]