Dogtrousers
Lefty tighty. Get it righty.
I'm not keen on helmet lights.
To me, they don't present a steady image of a cyclist. When the rider turns their head, the light to the car behind can momentarily disappear.
Off road is a different case, as the light points to where you look, which is a bonus.
Not to mention the fact that, if you read the small print leaflet that came with your helmet, it's very probably going to have a section in it that says "do not attach anything to this helmet". Plus the fact that they are extremely antisocial if you happen to forget to switch it off and then talk to someone while facing them.
.