Rosco and antihero are both partially right IMHO. A tactiacal light should be used to identify and locate targets. The rest of the time it should be off because it IS a target.
The surprise a BG may get is a nice bit of icing on the cake. However, if the light was on too much previously, he won't be surprised, but waiting. Very bad! So a momentary switch is essential.
A constant on switch is also essential because once a threat is identified the light should never leave it until that threat is neutralized or a greater threat becomes feared. This is also one of the weaknesses in most tac lights available - BG gives up and surrenders immediately, but the police are still ten minutes away! If there is enough ambient light, the threat could be covered and the light turned off, but this leaves a lot of "what if"s.
My tac light training indicates momentary use until threat is identified then a switch to contsant use until the situation is resolved.
Another theory is to work in a team with constant light use. In this way, quick movement and huge amounts of bright light can overwhelm a threat. Just imagine two or three of those milleniums or such lighting up your world all of a sudden.
Pretty hard to pick out people behind them let alone trying to start a fight. OTOH I'm not totally sold on this one, I picture some guy with a sawed off twelve guage or a full auto uzi just waiting for those pretty lights.........
