I used to fly a Pitts and thought I might be able to answer from memeory, but I can't, too long ago.
Anyway, from
howstuffworks.com,
The first is the flop tube design used in my airplane, a Pitts S-1T. The fuel tank is located in the fuselage in front of the pilot's knees, and inside of the tank is a flexible hose with a weight attached to the free end. When the plane is right side up, this hose, or flop tube, 'flops' to the bottom of the tank because of the weight and draws fuel from the bottom of the tank. When the plane is rolled to inverted, the weight causes to hose to flop to the top of the tank (which is really the bottom now) and draw fuel from there. This is really a cool design because it uses only one tank, and you have access to all the fuel in the tank whether you are right side up or inverted. This design is used on all the high-performance aerobatic airplanes with which I am familiar -- these planes all have a fuel tank in the fuselage.