Pylon, or mast, rock occurs in twin bladed a/c under certain load, torque and airspeed combinations. It is a lateral oscillation of the transmission on its mountings which can, in some cases, become divergent. It is usually seen at low speed and high torque such as on takeoff, and is even more common when underslung loads are involved. Often exacerbated by worn mounting dampers and an 'overcompensating seat-cyclic interface' (Agricultural pilot!). Some AFCS fits can compound the problem and will have 'anti mast rock' circuitry built in. Normally cured by relaxing on the cyclic whilst lowering the collective slightly.
That's the practical side, not too sure about the mechanics of it but would supect 'Hooke's joint' effect had a part to play.
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