When someone pushes his bike beside him in a constant (low) speed the light will not be at constant (low) brightness, instead it will flash with it's full brightness.
Why does it work like that?
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This happens because the power from a hub dynamo (which technically is a magneto, not a true dynamo) is not clean sine-wave AC but consists of short pulses with alternating polarity. At high speeds these pulses follow each other fast enough that a LED can burn continuously with a small capacitor and a filament bulb does not have time to cool down between pulses.
The effect does not have anything to do with programming or the fact that LED is a diode, and it happens with filament bulbs too. A more complex LED setup could have a capacitor to enable continuous dim light and perhaps even be programmed to flash at low input power, but I doubt this is the case.
If there is a super-capacitor in the system it will need a full charge before permanent full brightness. The electronics may be programmed to switch to bright flashing mode at low voltage and low rpm/speed before the full charge at normal operating voltage and high rpm.
A bright flashing light it is more visible than low brightness permanent light.