Dash cruise Button simply makes the cruise control lever effective.
When active (speed is set and maintained)
Holding lever up increases speed, releasing the lever re-engages the cruise at the higher speed.
Holding down temorarily disenagages cruise, slowing the car, releasing the lever re-engages the cruise at the lower speed.
Pulling back deactivates cruise.
When inactive:
Pushing down sets cruise speed and engages cruise.