Basically, you fill a captcha once, and that gives 30 anonymous one-time-use tokens which are stored on the browser. The cryptography used ensures that there's

Basically, you fill a captcha once, and that gives 30 anonymous one-time-use tokens which are stored on the browser. The cryptography used ensures that there's no way to associate the one-time tokens between each other or back to the original captcha. Redeeming the token proves that you've already filled a captcha, and will bypass the captcha for that session.