HSTS tells a browser to use https for this host from now on, and the preload list puts that instruction into the browser before the first visit. Both depend on details that are easy to get almost right: the header has to appear on the bare domain over https, carry at least a year, cover subdomains, say preload — and the first redirect from http has to stay on the same host. This check asks the four addresses that settle it.
From this server: four HEAD requests, one per address. Redirects are not followed, no page body is downloaded, no cookies are sent, nothing is stored beyond the rate counter.
Limits worth knowing: this checks the requirements, not the list itself — whether a domain is already in the preload list can only be answered by the list. Subdomains are not enumerated either, so includeSubDomains is read as a promise, not verified against every host.