Thought I'd chime in.
Horses often get cinchy, bloat, nip, etc, from people cinching too tightly. If your tack fits, and you don't have too much pad under your saddle, you really don't need to cinch up very tightly. I have watched people really haul on that latigo and they just do not realize how much of a multiplying effect a couple loops through the rigging makes. Even a small woman or a young boy can cinch a horse way past what it needs to be.
Three things I have seen, besides inexperience, that cause over-cinching are a poorly fitting saddle for the conformation of the horse, too wide of cinch, and way too much saddle pad. Those big ol' pillow-like saddle pads, and some of those really expensive gell-filled and other kinds of "special" pads some folks seem to love, lead directly to over-cinching, as do those extra-wide padded cinches.
If your saddle fits the horse and the horse has decent withers, and you have a good wool-felt pad (I often use a 1/2" wool-felt pad with a navajo blanket cover), all you need to do is snug-up the cinch.
As for nipping, I use CowboyBob's method with the elbow when a horse threatens to nip, but hard enough that the horse doesn't want to feel it again.
However, if I actually get nipped or bit, I leave no question in the horse's mind that doing so will get him hurt. None of this slap him on the neck stuff. I will ball up my fist and knock him into next week if I can. There is no way I can physically injure a horse with a blow from my fist, unless I hit him in the eyeball, which I won't do, and I'm not a little guy. If I am working with a horse that habitually nips or bites, I will work with them on a lead (not tied solid to a hitch rail) and I will carry a leather bat or farrier's rasp in hand. When the nip comes I will instantly give the horse a very healthy wallop alongside the neck with it, and follow that up by aggressively charging into the horse and yelling, like an aggressive horse in the herd would do. It only takes once or twice for the horse to figure out he's not the herd leader and nipping the king is not a good idea.
Watch horses in the field together. Watch what happens when horses start nipping each other. They are not gentle and they INTEND to make it hurt. You should too.
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