Flap design question

I have been looking at some of the pics people at Oshkosh have been
posting and was wondering
about the design of the flaps on some of these planes. Instead of the
whole trailing edge of the wing
hinging down like most planes I am familiar with, only the bottom skin
of the trailing edge hinged down
while the top surface was stationary. I was wondering what the
advantages and disadvantages of this
flap design are.

--
Chris W

"They that can give up essential liberty
to obtain a little temporary safety
deserve neither liberty nor safety."
-- Benjamin Franklin, 1759 Historical Review of Pennsylvania