TITAN1833, the piston is a 2-stage, momentary contact switch. When the flashlight is off, push the piston a little way for momentary low level light, and further for high level. If you have turned on the flashlight continuously by turning the head for low output, you can push in the piston for momentary high output - great!
Because the plunger is slightly recessed from the back edge of the PD (which allows standing up on a flat surface), it is not so useful as a tactical switch as it is harder to find and actuate as compared to an external tactical switch. I also found that you really have to push in deep to turn on the high level, and this is a bit difficult if you are not holding the light just right. Adjusting Kilroy may help this, but I have not tried to do that.
Finally, on mine the low level output has quite a bit of blue tint, but on high - wow, very nice, white, and bright. For this size of flashlight with a smallish head, the output and beamspread is perfect. A great light with outstanding illumination out to about 50 feet, diminishing utility beyond. There is a lot of technology in this light, and it is built like a tank out of all the best parts and materials. Now I know what all the fuss is about! Quite simply, outstanding!