From Keeley:

Our 30ms Double Tracker is a very unique pedal. You can use the 30ms to achieve incredible studio double tracking effects previously only available with complex studio techniques. The pedal will reproduce your signal in all of it’s detail, and then you can tune those mirror images off by a slight amount, and delay it from zero-30ms to create double tracked “space” in your sound, almost like a great chorus pedal that doesn’t modulate or “warble”. You create incredible depth and thickness of the sound!  We’ve also included a bonus “slapback” echo mode.