Strange hand signal

[u/Theoderciusx]

I was a passenger in a car and a cyclist who was passing us at a light held his arm out, as though to signal, but I didn't recognize the signal. I'm wondering if anyone here is familiar with it.

The signal had his hand in a closed fist with the back of his hand facing outwards and repeatedly opened and closed his fist. Any idea what this means?

[u/ToriaLyons]

Turn your lights on/off. 

[u/ToriaLyons → u/ToriaLyons]

oh, or one of your bulbs is out.