text: It is called Bernoulli's principle.  As velocty of a fluid increases, the pressure decreases.  Think of a traffic jam.  As the traffic gets slower, the cars are very close together, but after the site of the accident, the cars speed up and the distance between them is very great.  Fluid molecules are the same, high pressure is an area of high density among particles, they're close together.  Low pressure is an area of relative low density, more space between particles.  It's the same principle that allows airplanes to fly.