It ends up being a lot like FIDO or Passkeys but without having to store a separate key for each site. Each key is derived from your master key and the domain so they are all unique, to prevent tracking, but you still don't have to save a separate private key blob for each site. There is also a recovery key built into the spec so that if your master key somehow gets out, you can use your recovery key to prove you're the real person and regain your account to change the signin public key.