I don't really understand the rationale behind this, either, and I'm even less an aerodynamicist than you, Graham, but I also recall reading about getting the B-24 'on the step' a couple of times from some articles or monographs on the Liberator.
From the B-24D pilot's manual:
'After reaching cruising altitude, level off- get on the step, and pick up speed before power is reduced to cruise settings. If power is reduced too soon and before ther aircraft has picked up full momentum for cruising, it would mush along in a high drag, high angle of attack attitude in trying to regain speed under reduced power, and would probably be quite sluggish. Approach the cruising condition from the top- both speed and altitude; never from the bottom!' (I have NO idea how or why this procedure was recommended!)
From Wikipedia on flying the B-24:
'I was necessary when flying the B-24, to get "on step". This meant climbing to about 500 ft (150 m) above cruise altitude, leveling off, achieving a cruise speed of 165–170 mph (266–274 km/h), then descending to assigned altitude. Failing to do this meant that the B-24 flew slightly nose high, and it used more fuel. '
My only thought is that this procedure might have something to do with the characteristics of the Davis aerofoil.... basically, you got me, my friend! (Is this a great hobby or what?)
Mike