This is a soft start module. It will reduce the starting inrush current to a reasonable value. Because (1) transformer will draw a very heavy current at switch on, until the magnetic flux has stabilized, and (2) at power on, filter capacitors are completely discharged, and act as a short circuit for a short period of time. The power circuit has few small value resistors (limit the high current) in series for first second, and then shorted after. It works for most power supplies from 600W to 1500W.
Vin = 120Vac / 240Vac 50/60Hz
Imax = 12A