By a simple means of low MAP sensing (closed throttle) and configuration interconnect if it sensed that the aircraft was in the "landing configuration" with the wheels up, it would put them down "automatically" for the pilot.
From memory the PA-28R had an additional mast on the left side of the fuselage to sense airspeed. If auto's enabled and speed too low....wheels go down (and also speed too low wheels won't come up).
One reason quoted to me for it's removal was the possibility of debris dumped by the pilot thru the storm window (cig butts etc) may catch and block the inlet causing a gear extension at crz speed. Dunno how likely it would be, but.....
Most of the Piper singles I've flown have had the "feature" removed.