How does the speedlight know that Canon is not sending the trigger flash?
Is there some kind of radio signal from my Canon camera telling my Canon speedlight to go off? Then why can't it work without the flash? Because there is no radio trigger, you have to pay extra for that, and it only works for the 600ex rt.
So how does it know?
And how the heck does it read the TTL information?
side note: Girlfriend uses Nikon (has commander mode). Yongnuo speedlight works as a slave for both our cameras (doesn't have TTL).
Read more:
Why can't I use a Canon speedlight off camera as a slave to a Nikon's optical trigger?