Compacting is supposed to represent a mastery based approach to material. It is supposed to allow for skipping mastered material. Functionally different than accelerating through a curriculum which may be structured with intentional repeated material.

I like this quote from a seminar doc about compacting:
"Curriculum Compacting might best be thought of as organized common sense, because it simply recommends the natural pattern that teachers ordinarily would follow if they were individualizing instruction for each student. "

http://www.gifted.uconn.edu/sem/semart08.html