errrrr.... what happened to the AA cooldown thingy...?

Came back after a very long hiatus and the AA cooldown doesn't seem to be the AA cooldown anymore. I don't get it? I don't understand what it does, even after reading the tooltip. I also don't want to put a ton of effort into understanding what it does. I just wanted to get into a game and throw shells at enemy ships. I tend to play a lot of cruisers when I play, especially from the US, so that AA cooldown used to be kind of a big deal. Is it even worth using anymore? Can I still just hit the button when enemy aircraft come near?