This is what it actually does: Instead of only letting the provider bear the cost of content hosting (traffic, storage), the client also bears costs when accessing in form of computation. Basically it runs additional expansive computation on the client, which makes accessing 1000s of your webpages at high interval expansive for crawlers.
> Anubis uses a proof of work in order to validate that clients are genuine. The reason Anubis does this was inspired by Hashcash, a suggestion from the early 2000's about extending the email protocol to avoid spam. The idea is that genuine people sending emails will have to do a small math problem that is expensive to compute, but easy to verify such as hashing a string with a given number of leading zeroes. This will have basically no impact on individuals sending a few emails a week, but the company churning out industrial quantities of advertising will be required to do prohibitively expensive computation.