> How's this work? (detailed) We send your browser a unique string. It must be "solved" within 5 minutes. You browser is told to hash that unique string over and over, each time adding a new number. Your browser starts at 1 and counts up. Doing this until the outputted hash starts with 5 zeros (this is the solution to the captcha and is needed to submit the form).
and why couldn't a bot do this again?