yikes! The docs had an overhaul last week, that link was removed as part of the overhaul. Should have paid more attention. Should be fixed now :)

> A one-sentence describing exactly what it does and how it works would help ;-)

Here goes nothing:

mCaptcha is a privacy-focused, highly accessible CAPTCHA that uses Proof of Work for rate-limiting. It makes the bot/spam spend more time and work sending the request than it takes for the server to respond.