I read it in the wired.com article about this also submitted to HN: "And [Vinay] Shet says even the tiny movements a user’s mouse makes as it hovers and approaches a checkbox can help reveal an automated bot."

Sounds like if I 'Tab' to the field & hit enter I must be a bot....

I guess it's a lot more complicated than it looks at a cursory glance.

It's almost as if Google is playing its cards close to its chest to avoid tipping off people intent on defeating it's captchas!