Credential leak detection is one of the techniques the extension uses to detect shopping site skimmers. When your browser sends a request, the extension checks to see if you've entered any sensitive data (such as a credit card number or a password) into the web page. If you have, the extension inspects the outgoing request. If the request is sending the details you entered to a different domain (for example, from a netcraft.com web page to example.com) then this indicates a potential skimming attack. The request is blocked to protect your private information and you are shown a block page notifying you of the attack. The infected page is also automatically reported to Netcraft.