I inherited this neat machine and it seems to be in very good condition. This machine has options to control Frequency, Volts and Amps.
There's a whole science on how and why this works and the best answers probably are on Google.
This type of therapy is often associated with massage therapy and. as I understand it, great benefits can be derived by electrical stimulation. Again Google would be able to answer more questions than myself but you can contact me if you want.