Complex sentence: The Getting Things Done method rests on the idea that a person needs to move tasks out of the mind by recording them externally, so the mind is free from the job of remembering the tasks that need to be completed.
Split into simpler sentences: The Getting Things Done method rests on the idea that a person needs to move tasks out of the mind by recording them externally. So the mind is free from the job of remembering the tasks that need to be completed.