How does this overflow work?

[urbaninja]

Im planning on building a wet/dry filter in the near filter but i have never used one before and dont wanna risk drilling holes in my tank (acrylic) so does anyone knows how this diy overflow works?
http://www.monsterfishkeepers.com/forums/showthread.php?t=144079

 

[CHOMPERS]

It doesn't. That one is a failure.

It was properly redesigned later in the tread. It is the same as the one in the sticky thread but this one was laid flat.