Every site answers something for an address that does not exist, and what it answers decides whether anyone ever notices a broken link. A 404 is a clear no. A 200 with a friendly page is a yes, and search engines treat it as a page worth indexing. This check requests three addresses that cannot exist and compares each answer with a page that does.
From this server: one request for the given page and three for random addresses on the same host, each capped at 256 KB. No cookies are sent, no scripts are executed, nothing is stored beyond the rate counter.
Limits worth knowing: the three test addresses are random, so a cache cannot answer them from an earlier visit. Redirects are recorded but not followed — what matters is the first answer, because that is what a crawler records.