The second finger (third digit) has the most literal meaning of all. Less commonly referred to as the long or tall finger, the digitus medius manus sits in the center of the hand, right between the digitus secondus manus and the ring finger. How the middle finger became an offensive gesture is another story altogether.
To see how the pump works, look at the drawing below. The pump consists of a small diameter pipe (called the piston) inserted into a larger diameter pipe (called the cylinder). The drawing shows the pump at two points during its operation. The top drawing shows air being drawn into the pipe by pulling out the piston. The bottom drawing shows air exiting the pipe when the piston is pushed back in. At the right end of the pipe is a commercially bought one-way valve.
When it does cool down, the pressure will then drop to less than atmospheric pressure, so you'll have a partial vacuum. Probably the easiest way to create a vacuum at home is with a suction cup. If you press a suction cup flat against a wall and pull back, the inside of the cup will contain a vacuum. (This is why the cup sticks to the wall.) You can also create a vacuum inside of a syringe. If you seal the syringe up and pull back the plunger, there will be a vacuum inside.