Page 76 (12-13).
Two Points:
One: The עבודה of רצוא ושוב is explained in two ways:
1) It is necessary for the person to put aside his selfish interest for the ideal interest of the creator who desires דירה בתחתונים.
2) The cycle of movement helps deal with the lack of complete efficiency (התיישבות) in the relationship between Godliness and the person.
Two: The Rebbe introduces the beginning of the next new big idea, every lower world responds and runs towards G-d because of the world above it, yet somehow the lower level runs with more energy than does the higher level.
