It is called a vertical expansion slot, there is nothing different other than the regular slots being horizontal. It is used for extra accessories ei usb, esata, fan controller, etc. These are the same exact things that can also be placed in the regular slots but would need an extension wire to reach. It is only a single vertical slot so would not hold a dual slot card.
So if I used a dual slot video card that covered a PCIex1 slot, could I connect an extension under the video card so that slot could still be used?