The vintage Magnatone amps were known for having that amazing wobbly sound, from discreet low intensity effects that makes an instrument mix well into the sound of a band, to extreme effects that can take the sound of guitar out of this world! Depending on settings, it can be deeply bluesy, very modern or spooky sounding.
The SurfyVibe circuit is emulating the mono channel of a Magnatone 280. The "sweeping" mode adds a feedback path to create a unique effect that is a mix of phaser / chorus / tremolo.