[jolmg]

As examples of reviewing in different manners, you can "review" by applying that knowledge or learning on top of it. Like you can read about an alphabet, then review by practice writing it, or review by learning words that use that alphabet. In that way, "reviewing" is not a waste, as you can gain new knowledge or experience. Spaced repetition is merely trying to improve retention by putting increasing spacing between the recollections of that knowledge.