Just so we don't all seem so vicious, let's note that this design is actually meant as a non-lethal alternative to drawing a firearm - provided that the little button near the head of the flashlight is "tactically correct" - i.e., press it, and the light flashes on; let up on it, and the light goes out.
If that's the case (and I'm betting it's not) then the use of the light is to hold it in your weak hand, at the shoulder, with the pinkie finger nearest to the head of the light, so that the shaft of the light rests on your shoulder.
This leaves your strong hand free to draw your sidearm.
The pinky finger actuates the light. And it doesn't take a lot in a dark space to "blind" an opponent with a flash of light. Moreover, if the light is on your shoulder, it's harder for the opponent to see you - particularly your face and your strong hand. Turn at a slight angle with the strong hand away from the opponent to increase this effect and present a smaller target.
Since the light is now held on the shoulder like a club, simply extending the arm quickly lands a blow of the light's shaft on the opponent's head or shoulder/neck area.
If the light in is your left hand, that means you'll be striking a very quick, very powerful blow to the opponent's right side, which is most people's weapon-bearing (strong) side. Even a sloppy downward blow the shoulder near the neck can momentarily disable the arm.
In more tense situations, with sidearm already drawn, a tactically correct light can be held in the weak hand, with the back of the weak hand braced cross-wise against the back of the strong hand holding the sidearm, creating a sort of modified Weaver-stance, two-handed presentation of the sidearm. Using the pinkie, the switch can be actuated, using the flashlight as a sort of poor-man's sidearm-mounted flashlight, an item which is, itself, often offered at Woot.