I would assume the cookie is still valid and you're authenticated, or you type the email address and you get a fresh token.

It would be based on cookies, no different than logging in with a password, then returning at a later date. Instead of typing a password, you click an e-mail link. That's the only difference, and like I said, it comes down to preference.

A number of people have said it provides weaker security, but I disagree. You can guess passwords, you can record or watch someone type a password, or catch the password over insecure protocols. The e-mail approach doesn't have these holes. The link is valid once, so if you login and someone else tries to use the same URL after snooping, it's not possible. With password logins, people can use the password for that site, or your e-mail password to reset the login. With the e-mail method, they need your e-mail password, that's the only way they'll be able to login to your account.

However, you do need secure access to your e-mail. I know there are times when I'm on a public or friends computer, and I need to quickly login to a service. I'd much rather enter my password only used on that service, instead of entering my e-mail password. This way, if it becomes compromised, it would be a much smaller concern.