How does Stack Exchange log you in when you're logged into Stack Overflow?

My guess is: SO includ开发者_StackOverflow社区es some sort of <img src="stack exchange URL" /> right when I log in with a one-time token or something. Can somebody tell me how it actually works?

The Stack Overflow blog post on Global Network Auto-Login explains how this happens. Essentially it uses HTML 5 storage combined with a central authentication site at http://stackauth.com.