Preinfusion is one of those irritating terms that everyone seems to use differently. I think that static preinfusion basically means that the water is allowed to saturate the coffee bed at line pressure, without the pump running. For this, the machine must be plumbed in.
As for how it works, I guess that a better question is whether it works. Legions of people out there seem to be of the impression that preinfusion is a good thing. Im not so sure. Some of the best shots that I have had have started pouring three seconds after the pump was engaged. Many say that apparently it is more difficult to pull consistent shots without some sort of preinfusion.
Then we get into the question of whether line pressure preinfusion itself is a good thing, as opposed to any other type of preinfusion. Many machines use a gicleur/restrictor to slow the build up of pressure with the pump running. Some people insist that this is not preinfusion. Regardless, we have an easy way to compare the two in the Synesso Cyncra. This machine has both a gicleur and a three position paddle. The paddle has an off position, a middle position that allows water to flow at line pressure without engaging the pump and an on position. The idea is that the middle preinfusion position allows you to control the length of the preinfusion time. The fact is that I have never seen anyone actually use the middle position; in cafes, the lever always goes fully on and the gicleur takes care of the pressure build up. Perhaps the middle position would be more useful if there were no gicleur.
Hope that helps,