Whistler precursors are discrete rising tone emissions that precede two-hop whistlers, starting shortly (0.1-0.3 s) after the one-hop delay. New data from Siple Station, Antarctica, and Roberval, Quebec, are introduced suggesting that precursors start at power line harmonics (PLH) from the Canadian electrical power system. To explain precursor triggering, a new two-stage mechanism is proposed in which the upgoing whistler interacts through longitudinal resonance with energetic electrons, thus modifying their distribution function. These electrons then interact through cyclotron resonance with the backward traveling PLH waves, causing these waves to grow to the level where they can trigger precursors. The PLH thus acts as a kind of catalyst in converting a particle perturbation to a backward traveling wave. This explanation, if correct, opens up a new realm of study in which whistler mode waves can be used to create a temporatory perturbation in the energetic particle distribution. Such control is needed to conduct experiments on the factors (anisotropy, flux) that affect wave-particle interactions in the magnetosphere.