It is just due to css effects.
The div which contains all the login controls is hided by default and is present just below the "Sign in" link.
When you mouse hover to the "Sign in" link, it comes out below the link as a box because there is a z-index and opacity set to that box. So, it comes over the page.
If you want to check, then just view source of that page and check the css and html in FireBug and you will know how exactly it is implemented.
Else check
here.
Submit an article or tip