When I was in the South African Air Force I had the opportunity to march in a military tattoo. There were about 120 Air Force marchers lined up in rows of three. We drilled several hours a day for many weeks. Finally, we got to the military tattoo. It was marvelous to watch 120 Airmen dressed in blue uniforms with white spats, white gloves and white belts marching in perfect unison. There was an incredible symmetry and harmony as we went through our drills that evening.
The secret to our marching was the lead person in the formation. He was in the front right. Everyone took their cue off that lead person.
Paul is saying that now that we live by the Spirit we are to take all our cues off the Spirit. We are to watch the Spirit; we are to follow the Spirit; we are to keep in step with the Spirit.