Lines that flow

Was looking through the fourums and saw this link and wondered how they did those lines and made them flow liek that (the fist thing you see, and during transitions)

At first I thought it was just a mask, but then when you see the lines cross over each other there is no way it could have been a mask.

Maybe its an element that follows a path, drawn in some other program??? i saw this same effect in the site of the week a last month