Trick is to have the light held with some kind of rubber band type thingy, so that in an impact, it just breaks off sacrificially. But I guess, in the unlikely event, if the impact is at just the wrong angle, it just drives through the impact-absorber. But I don't think it's possible to fall straight down (negative Y speed, but no X/Z speed vector) when travelling at speed.

At least that's how my bicycle helmet light works.