The idea is that a tuned circuit is set to some frequency and driven with twice that frequency. A parametron is essentially a resonant circuit made with inductors or capacitors whose reactance is varied at twice its resonant frequency. Then again, the FACOM used 5kW of power, the PC-1, 3kW, and the PC-2, 10 kW, so there is that. There were some other uses, including numerical control of machining devices, a Morse code to Teletype converter, and even a prototype system that used parametrons to sense cars passing over a road. While it is just a footnote now in the history of computing, a significant number of Japanese computers used the technology.