What are drifting valves anyway? When a locomotive is coasting, even though there is no steam pressure acting on the pistons, the pistons are still moving, alternately creating pressure and vacuum. The drifting valves even out those pulses.
"Drifting valves are usually so arranged that a jet of saturated steam is supplied automatically to the steam chest when the throttle is closed and the engine is drifting."
-Locomotive Management, 1937.