What should be noted here is the exceptionally fast “rise time” and “fall time” (also known as “decay”). Rise time is the front (or leading) edge of the square wave. The fall time is the trailing edge of the waveform.
In a theoretical world, the rise and fall times would be 0, or instantaneous. The square waveform is chosen above all others because of this unique time characteristic. It is believed that most pathogens mutate and eventually become used to or immune to waveforms with slow rise times or decays.
Square wave, having a sharp rise and decay, is believed to be more effective and harder for micro-organisms to adapt to. Thus, square wave is used for killing pathogens and sine wave is used for regeneration and healing. It should be noted, however, that square waves have also been used for healing and found to be effective.