I say you flap when the killer is near you so people need to stay away. No flapping means it's safe to save.
If you see a flap, you'll get a slap.
If arms are still, you won't get killed.
Do I have it backwards?
Nah you have it right, you flap when the Killer is next to you, and you don't when they're not.
Personally, I don't think anyone actually follows that sort of thing though... since I just flap my arms whenever, and people still come to me no matter their rank...
I think its just a meme to flap honestly.
As long as I can remember, the flapping arms is a signal that the killer is nearby.
So as far as I'm concerned it's always been the universal signal for "I'm being camped, stay away"
I use it that way too. But there isn't a 100% confirmation that another player will use it that way. I noticed that newer people usually do the opposite. Flapping the arms to say "come get me now, please".
There isn't a right or wrong answer about this sadly.
You can organize during the pre-match lobby though (if you're on PC) to specify it so luckily people will use your strategy (or the opposite if you agree on the other one).
I usually flap them when the killer is coming to me and stop when they leave.