Typing your address lets Chrome ask your mailbox provider, behind the scenes, whether you're currently signed in there. If you are, it quietly stamps a hidden field on this form with proof of that - nothing is typed, copied, or clicked. Submitting the form just hands that proof to this app's server, which checks it against the mailbox provider before starting your session.
Chrome only, and only for builds enrolled in the origin trial - regular stable Chrome won't do anything special yet.
Gmail is the only provider live on the protocol right now, so non-Gmail addresses are expected to fail verification here.
Built by Kamil Marczak (GitHub) on top of better-auth-evp (npm), a Better Auth plugin implementing this flow.