Here's what EQD's website says about the effect:
Vintage spring reverb emulation capable of providing a little atmosphere all the way through to vast depths in one tiny box. The attack control sets the delay of the initial pick attack on the wet signal only. When playing staccato this registers as a slap back type echo hitting the reverb but when playing fluid lines it adds a massive depth and “fattens” up the sound. The dwell control is a fine tune function that enhances the decay and the depth controls the intensity of the reverb.
Update: If you're having a hard time getting unity volume, try replacing the 10k resistor of pin 9 of the TL074 just above the 2.2uF cap with a 17k resistor (or something close to that).