Comes on and stays on. I presume that the light has some system for allowing the capacitor to power the light only when the voltage from the dynamo drops below a certain level.
@mespilus there's only one (COB) LED for both running and standlight functions. No daylight sensor, it's a fairly simple light. Here are pictures of the board. The wires to the LED are the red and black ones at the top and bottom of the board. The pair of red and blue go to a 0.1F capacitor, but the big 1F coin-shaped capacitor under the board is the one that, I think, actually powers the standlight function.