The Arisaka Momentary Tailcap is a replacement switch for the Arisaka 300 and 600 Series Light Bodies, plus Surefire Scout lights. This model is designed to be momentary only with twist for constant on. The tailcap can be threaded all the way in for constant on, or backed out approximately two turns to prevent accidental activation during storage or travel.
Momentary only switches are the preferred style for many weapon instructors because they simplify light operation. Under stress, there's no need to worry about accidentally pressing the switch too far and clicking on the light (like with a Surefire Z68 or Arisaka Clicky Tailcap).
Thread the Momentary Tailcap onto the light body until the light activates. Back the tailcap off 0.5 to 1.5 turns until your preferred amount of switch travel and feel is obtained. The light will activate as long as the button is pressed.