Generally, when you tighten the tailcap all the way down, then back it off to the point where the notches align, the light will not activate from pressure on the body of the tailcap, but will from pressing the tailcap switch button. It's technically a good medium setpoint, because it's about halfway between locked out, and constant on. On some lights you will still have to back it off a little to keep from activating by pressure on the tailcap.
It does make it convenient to index the switch from the lockout position for use without actually turning it on. SF also has polymer holsters that can carry their lights bezel up, and downward pressure on the light shouldn't accidentally active it with the notches aligned or slightly past that point, but it will be ready to use when the switch is depressed.
I find it most useful on my new L1, which has a notch and a circle in the middle of the tailcap body instead of a corresponding notch. When they're lined up, there's no accidental activation from pressure on the tail(I got one of those tailcap shrouds, so this works beautifully), plus the switch is just a quarter turn from constant on in low mode.
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