The light comes with a cylindrical tube about an inch long that fits into the hole at the base of the light. You simply slide this into the hole and it activates the light. Levels are adjusted by nudging the tube while it is inserted in the hole.
Unique and effective. Reality is I just want to get my hands on the 1200 lumen model that Rob is selling and cannot justify another £275 on top of the £250 I paid for this light 3 weeks ago!