On the downstroke the wing stays extended and nearly rigid at its full angle of attack, so it shoves air down and back — producing lift and forward thrust at once, like a propeller blade.
On the upstroke the wrist bends and the primary feathers separate and twist, sharply cutting the wing's area and effective angle of attack. This asymmetry keeps the upstroke from cancelling the downstroke's lift.
This 2D companion mirrors the 3D simulator's blade-element model at a single representative wing station: the effective airflow angle comes from combining the forward flight speed with the wing station's own flapping velocity, and lift/drag are computed from that combined airflow using a stall-limited lift curve. At the small chord and modest airspeed of a real bird, the flow sits in the Reynolds-number range (~10⁴–10⁵) where thin, cambered wings are far more efficient than a flat plate — the wingtip Reynolds number above tracks how the current controls land in that regime.