What is the purpose of making a Yagi's parasitic elements either longer or shorter than resonance?
Why A parasitic element has no feed line; it is excited by the field from the driven element, and the current that flows in it depends on its reactance. Making an element longer than resonance makes it inductive (lagging current) so it re-radiates as a reflector, while making it shorter makes it capacitive (leading current) so it acts as a director. Those phase shifts, combined with the spacing delay, cause the re-radiated fields to add in the forward direction and cancel toward the rear, producing gain and front-to-back ratio.
Watch out Loss reduction is tempting because off-resonance elements sound inefficient, but element length here is chosen for current phasing, not efficiency; mechanical balance and wind torque have nothing to do with electrical tuning.
Long = lagging = reflector behind, short = leading = director ahead. Length sets phase, phase sets the beam.