I'll have to test if when the light is turned off through the computer, the radar still flashes when a car approaches. The radar works fine with the light off but I have never thought to see if it flashes. I'll test it tomorrow.
I'm not sure I see the merit in this. The radar range is about 150 meters, but the light is visible from a lot further away than that. I would prefer drivers see me from a long way away, rather than a sudden and unanticipated light. YMMV.
I haven't seen it myself, but am assured by my riding buddies that from its regular flashing pattern that the light "goes crazy" when a car approaches.
With day flash, the battery life is 15 hours, so it's not as though you are saving a whole lot of battery life through having the light off.
Think of it as a group ride mode, and then it might make sense. Some people become irate when you use lights in a daylight group ride. Yeah I don't get it either, but I think it would be nice to have it as an option. Perhaps the Varia radar itself has the resolution to detect the following conditions:
1) Car behind - flash on
2) No car behind, no cyclist behind - flash on
3) No car behind, cyclist behind - flash off or low intensity