The hostname of the website you're visiting is cryptographically signed by the security key. So, if by accident you're on phishing.com, the key will generate a

The hostname of the website you're visiting is cryptographically signed by the security key. So, if by accident you're on phishing.com, the key will generate a signature containing phishing.com. When the attacker forwards it to google.com, Google won't verify the signature, and thus the attacker won't get access to your account.