Captcha works if it’s a newer AI version but the old CAPTCHA systems are all easily cracked.
Anyway short version is that they look at all kinds of behavioral inputs with newer CAPTCHA systems including mouse speed, movement, timings between clicks, resting cursor over things, etc. They build profiles off of humans and bots and use for comparison etc.
For example if you look at your mouse cursor right now, try to move it in a perfect straight line for a while - you can’t, it’ll be off a few pixels here and there. They look at stuff like that and a million other things as you move around.
The more input they have the better which is why older CAPTCHA dialogs on their own aren’t actually great because they don’t gather a ton of input and behavior.
With a game, Blizzard has an absolute massive metric ton of inputs as you play so in a sense the game can be kind of a giant CAPTCHA system on it’s own - CAPTCHA meaning ‘Completely Automated Public Turing test to tell Computers and Humans Apart’.
They can embed roughly the same CAPTCHA code in the game as you play looking at inputs and behavior, timings, how random it is, human-like, etc. But to not be defeated they need to use machine learning or reinforcement learning AI that adapts and improves as it goes so it doesn’t get defeated.
It’s an arms race but it’s an arms race that ends up being very costly for hackers and I don’t know if hackers are going to go to the trouble of dealing with AI on their side.
Summary is that the newer AI CAPTCHA systems are invisible to the user and are non-invasive but I doubt Blizzard will go to the expense but who knows.