a cinch goes from one side of a saddle, under a horse's belly to be hooked to the other side of the saddle. If ya don't know what yer doing, a horse will take a deep breath and blow out their belly so that yer cinch seems tight, but once you put yer foot in the stirrup and swing a leg up, the horse lets the air out, the saddle slips and yer most likely face-down in the dirt. A wise horseperson will brace their foot up against said belly, tightening the cinch while *encouraging* said horse to let the air out and get the cinch good and tight.

My grandmother was a schoolmarm who rode a horse to school. One day a young man, hoping to impress her, offered to saddle it up for her. She allowed as that would be alright with her. She pretended not to watch, but insisted he "gentle" her horse for her some after he'd saddled it. Well, he was the greenhorn she'd suspected and shore enuf he went under the horse, along with the loose saddle! She had quite a chuckle over that for years.

At least that's the kind of cinch I know of...though there is the old expression of ease as in "it'll be a cinch!"