Robdeanhove is going to be able to answer that better than I but once you’ve got up to speed then it doesn’t drop off and lose that much power if you slow down.
However if you stop or the first part of the ride is an uphill twisty slog then the light output won’t be as much as if you’ve done some decent speed before hand.
To be honest I haven’t taken it off road yet, but there seems to be plenty of light available from using it on unlit (pothole strewn) roads.
Pair with a helmet mounted light to get better visibility in the twisty stuff? So you can see where you’re looking rather than the bikes pointing?