The simplicity of it is logic defying. It used to be that you had to find crosswalks or move puzzle pieces or type blurred letters and numbers, but NOW all the sudden I can just click a box and HEY!, I’m human?
That’s hardly the Turing Test I’d expected.
I don’t know for certain, but I think it is simply looking at what you do with your mouse. If the movement is erratic, imprecise, and delayed it goes ‘yeah, that is either a cat that got lucky which is close enough or a human’. The reason I think this is that I’ve failed same site’s checks if my mouse just happens to be hovering over the checkbox when the prompt appears. Retry, move the mouse, success.