Set the pendulum A into oscillation by moving it to one side, normal to its length. It is noticed that pendulum D begins vibrating with a tiny amplitude and eventually attains the same amplitude as pendulum A. Due to the constant total energy, when the amplitude of the pendulum D increases, the amplitude of the pendulum A decreases. The amplitude of the pendulum D will decrease with time, whereas the amplitude of the pendulum A will grow. Only between the pendulums A and D can energy exchange occur since their inherent frequencies are the same. In addition, the pendulums B and C have relatively tiny amplitudes of vibration.
Through XY, the vibrations created in pendulum A are conveyed to the other pendulums B, C, and D as forced vibrations. The pendulum D enters a state of resonance, while the pendulums B and C continue to vibrate at a forced rate..