Oh...thanks I'll see if I can see one somewhere; I guess it'll be on the light unit somewhere..
It's on the bottom edge of the light and really is small - it hardly looks like a switch.
The light may also have a slightly bigger red button which is to 'dump' the charge in the capacitor which keeps the light on for a minute or two at rest.
Why would you want these features?
It's mostly due to strict regulations about not showing a red light in a railway station.
When you push the bike onto the platform, you can quickly push the red button to kill the light.
The switch is more to do with an occasional application on a touring bike.
In certain circumstances you might want to ride with just a front light so you can divert a bit more of the limited dyno power to charging a gadget.