Just your typical passive feedback loop. The LOOP switch engages the effects loop while the FEED switch shoots the output back to the input. The amount fed back is controlled by the one and only knob. The result can be aggressive, difficult to control and frankly, unpleasant. Perfect for the experimentalist in all of us.
Uninitiated should take caution. Feedback loops are hard to predict. Some pedals will render the feedback useless, others will allow unique sounds that would be impossible to achieve otherwise.
Experimentation is key. Type of effect, pedal placement and gain structure are all important parameters. Mix it up, but always beware your speakers and ears.