bottle dynamos tend to slip in the rain, lights don't need an off switch.

hub dynamos don't slip in the rain but they have an on/off switch.
[LED lights really never have to be turned off anyhow..]



Decades ago I used one of the BB dynamos, I could engage it while moving, with my toe on the lever..

back then some frame builders put a shift lever behind the seat tube, high up, to let it go against the tire or pull it away,
with the cable from that lever..





....