When the NUMB knob is fully CCW and the DWELL knob is past 2:00, the pedal will start to feedback. The feedback will continue until you either adjust the NUMB up or the DWELL down. Self-oscillating will overdrive quickly, but in a cool way.
Hold down the right switch to start a self-oscillate mode. NUMB knob chills out a slapback delay when turned up. DWELL is how much signal gets pushed to the reverb circuit. NUMB mostly CCW and DWELL mostly CW will get the most spacey.