Motorcycle headlamp modulator
A motorcycle headlamp modulator (or simply headlamp modulator) is an accessory device that oscillates the intensity of a motorcycle headlamp at 240 ±40 cycles per minute (~4 Hz) between approximately 20% and 100% of full intensity. The headlight operates at full intensity 50-70% of the time.
Source: Wikipedia — Motorcycle headlamp modulator (CC BY-SA 4.0)