>- The ripples in your diagram are caused by the instantaneous algebraic sum of the component sine waves.
I am well aware of that, but the squarewave can also induce ringing whenever something is present that can resonate with the square wave.
The point is that for the resonance to continue, the square wave must be in synch with the resonant frequency. This can be illustrated by looking at the same waveform illustrations.
From John Bedini: Tesla discovered several facts concerning the production of his effect. First, the cause was undoubtedly found in the abruptness of charging. It was in the switch closure, the very instant of "closure and break", which thrust the effect out into space. The effect was definitely related to time, IMPULSE time. Second, Tesla found that it was imperative that the charging process occurred in a single impulse. No reversal of current was permissible, else the effect would not manifest.