In “scripted battles,” (usually for the public) you are told what to do. In “tacticals” — private or “non-public” events, where the event is actually more of a tactical exercise, one actually uses the “honor system.” Not perfect, but it works.
Sometimes, hits are rehearsed — especially if it’s a “known” death in that particular reenactment such as a noted officer or figure that was killed in the battle. For others, the time to take a hit is when they get tired of charging across the field and need a rest — that’s quite common… this is, after all, a hobby. Have fun with it. And remember, you can’t have “reenacting” without “acting.”
Another deciding factor is when a piece of equipment fails on you, usually your firearm. If you’ve tried three times to get your flintlock to fire the same charge, the usual practice is to surreptitiously pour the charge out on the ground, feign reloading, and take a hit. I’ve found, howeve,rthat it’s generally better to take a hit as a wound: Not only is that more historically accurate in most cases, it also doesn’t leave you lying in the middle of a field in the hot sun, in an awkward position, with cramps setting in. If you’re “wounded”, You can always drag yourself to a shady spot, or to a nearby stream, which is what you’d do if you were really wounded, anyway. When I’m doing French Marine, I like to yell insults in French, at my friends on the English side! (hey, at least the Quebecois seem to get a laugh out of it!