From my experiences, any knot that's under the load of a scared, hurt, caught, whatever horse. Is NOT getting untied, nor is a buckle, hasp, or latch getting undone.
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Just as a "for what it's worth," I've had a horse break a 2" bull snap on its lead rope when it sat back hard. I was grooming a huge 17.2hh TB gelding who was built like a draft horse. Groomed his left side with no issues. Moved to his right side, touched his neck with the (soft) brush, and he absolutely freaked out. The bull snap broke on the top solid hook part clean through. Caught him by surprise, he went down on his hocks, and nearly took me down as he flailed to keep from going over. It all happened within five to ten seconds...not nearly enough time to even think, much less whip out a knife and cut him loose.
On the blocker tie ring...if used correctly, they are invaluable. My gelding didn't know how to stand tied when I got him. I strung his lead rope through the blocker ring and wrapped it around the hitching rail a couple of times. First time he sat back, it fed out a couple of feet (a few steps for him), but once the pressure was released, he stopped. Hard tying a horse that doesn't know how to tie or is prone to sitting back can be dangerous, IME, because if they're sitting back, they're most likely panicking, which means they aren't going to be "thinking" about "Oh, hey. If I step forward, the pressure from the halter will go away" (not literally thinking, of course, but I think you get what I mean). Instead they're thinking "I need to get away and this thing (the halter/pressure) is preventing that, so I must fight it to get away." The blocker ring provides an almost immediate release, so the horse doesn't actually get far before the pressure is released and they feel "safe" again. Posted via Mobile Device