Google collects all your data and applies machine learning to predict a probability value that you are human. If it is below a certain threshold you have to enter a CAPTCHA.

That's the question he answered. It's machine learning based on browser variables and mouse movements.

The old system presented you a challenge-response test no matter what.

Sorry, I just thought "all your data" was a bit unspecific. I have no idea what Google collects in order to derive that. I have a lot of guesses, but "all your data" doesn't tell me in fact, how it works. And I thinks that was what the parent poster asked for.