Maybe it's my imagination that's getting the better of me but when you say "zap" and "jolt" I get the image of a sneaky electrical shock to the butt when they're not being careful enough around the snake... or perhaps a loud noise to startle the dog. I don't see how that would actually cause the dog to associate the sudden startlement with an avoidance of rattlesnakes unless the snake itself was somehow delivering the jolt. Must indeed have a lot to do with the timing as you say.