Abstract: Three elements are hydraulically coupled by a hydraulic fluid contained within one of the elements serving as a housing so that reciprocally driven motion of one of the elements is transmitted to move another element responsively in a reciprocal motion. The movements can be counter-reciprocal or co-reciprocal, and the housing can be an unmoved element or can be one of the reciprocally moving elements. The stroke lengths and velocities of the reciprocal movements can vary, and the effect can involve dynamic balance, power transformation, vibrational dampening, and many variations of these possibilities.