I found the simplest way to put more energy into the downstroke was to tension the wings in the down position. By experimentation, the result is a slow upstroke with a build up of resistance in the tensioners, followed by a more vigorous downstroke. This simple adaptation leads immediately to a more realistic flapping motion.

A second important point is that the wing on the downstroke must either rotate bodily into a more leading-edge-down position (ie: negative angle of attack), or the wing membrane itself must be flexible enough to twist into this new position. The best way is to pivot the wing root so that it is able to rotate appropriately, and then tension it back into the positive angle of attack position. During the powered downstroke, the torsional effect of having the centre of effort of the wing behind the pivot axis causes the wing to tilt into a nose down position until the upstroke begins.