The button on top of the controller does 2 things:
Push it, and the light dims down to about 10% brightness. You can now inspect your plants and your eyes won’t get strained too much (still avoid looking into the LED).
If you push the button again, the light goes back to full power. Or, if you forget to push it, it goes back to 100% after 15 minutes.
If you hit the button twice in quick succession, the light will switch off.
Push it again, it turns back on.