In order for an agent to change things back in time, this agent must not know of the outcome he is tasked to change. Otherwise, his actions will just fulfill the original outcome without effecting any changes.
He must be ignorant of the series of events he is to change. He will then not experience any force along his timeline (his reality) as he effect the changes.
An effective time agent knows nothing. Once he is aware of the outcome he is to change, he is immediately rendered ineffective as he will experience tremendous forces along his timeline as he tries to change things.
Reality has mass along the time dimension. Changing reality, stop short many events along the time dimension. Each of these events, project a physical force in the space dimension. The change agent is literally pushed to fulfill the original outcome, instead of effecting the desired changes.
A change agent does not know of (nor experience indirectly) the undesired events he is to change back in time. Know this!
