There is no mystery to what Dr. Rife did. He simply connected a device called a signal generator to Petri dishes loaded with bacteria, viruses and live cancer cells. When he cranked up the frequency, at some point the cultures on the disk died. He watched this happen on his 60,000 magnification microscope. Different organisms perished at different frequencies. He compiled a list of what pathogens succumbed to what frequency, and so we have Rife's CAFL lists. Some of his equipment also produced harmonics of the base frequencies, and those associated energies may be beneficial.
The statement about Clark's units "would be better if they had no frequency at all" is incorrect. A flow of electrical current without any frequency is DC, or Direct Current. DC is not good for zapping because it promotes electrolysis, which can generate gas in body tissues. Any waveform above zero Hertz will not generate gas because the pulses are too short, but it will sting and kill pathogens.
The Thundervolt Zapper is partly a Rife machine because it generates a wide spectrum of frequencies. But it's Frequency Freeze mode targets pathogens and matches their frequency until the parasite dies. This match up is done automatically while the user zaps. There is no guesswork involved.