There is a small brass pin that protrudes slightly from it which makes contact with the top of the body tube first, before the entire ring comes in contact with it as it is screwed in.
This pin is insulated from the rest of the ring, and attached to a resistor, and the other end of the resistor is attached back to the ring, so that when the top of the body tube is contacting the pin only the voltage to the light engine is reduced. Tighten the head further and contact is through the ring itself and the resistor is bypassed, and the full voltage is transmitted to the light engine.