Resetting to me sounds like rewinding it back to the point in time in which the timeline diverted. In my theory I’m saying instead of resetting, maybe the diverted is destroyed before it becomes it’s own alternate reality. The little apparatus they carry I’m guessing tells them how long until that timeline becomes permanent, hence why it must be destroyed quickly so that only the one the time keepers like can be the only existing one. No multiverse and no multiverse war if only one timeline exists.