Tremolo is one of the first, oldest effects for electric guitar.
The Surfy Trem is a very clever, handy and small pedal that contains two very classic and musical tremolo circuits.
Those are the "blackface" bias tremolo as found in a Fender Princeton or Tremolux, and the unique "brownface" tremolo that was in the larger brown Fender amps of the early '60s – a unique sound that adds a mild vibrato and phase shift to the familiar volume fluctuations of a typical trem – an effect often called "harmonic vibrato" and somewhat similar to a univibe or a phaser, yet different.
The Surfy Trem has more range (speed as well as intensity) for its two effects than the '60s amps it's based on, and that means you can still make a classic effect sound different and deeper than the original, but without hum, noise, or the typical "motorboating" of an old amp.
The Surfy Trem runs on a standard center negative 9V power supply (like 95% of the effects pedals you know) or a 9V battery.