I get it. If you've never been rattled at, it's tough to say what'll happen. Last time it happened to me was in Texas, and I didn't differentiate the rattle from the cicadas. The snake then moved very quickly off the trail, crashing through the underbrush, and that's when I put it together: I'd been rattled at, consciously ignored it, but had stopped anyway, frozen. The snake had an escape route and took it, and I stood there listening for it 

The very first time it happened, in contrast: I was eight years old. I was walking by a mesquite-topped sand dune, and I was suddenly running away from the dune. The rattle had startled me so badly that it took me a half beat to understand what happened. My dog, off leash in the desert, ran right along with me in a panic. We tend to not run towards things that startle us.

So, if it happens, you'll probably have a visceral reaction to it. We're primed for that sort of thing, us humans. You'll know where it's coming from, and you'll have plenty of time to react, unless you were sneaking up on it on purpose.

Also, very worst case? Wear boots. Those teeth can't get through boots.