It was discovered by a lucky chance, and then developed into a governable form of propulsion by the Galactic Government's research team on Damogran.
This, briefly, is the story of its discovery.
The principle of generating small amounts of finite improbability by simply hooking the logic circuits of a Bambleweeny 57 Sub-Meson Brain to an atomic vector plotter suspended in a strong Brownian Motion producer (say a nice hot cup of tea) were of course well understood --- and such generators were often used to break the ice at parties by making all the molecules in the hostess's undergarments leap simultaneously one foot to the left, in accordance with the Theory of Indeterminacy.
Many respectable physicists said that they weren't going to stand for this --- partly because it was a debasement of science, but mostly because they didn't get invited to those sort of parties.
Another thing they couldn't stand was the perpetual failure they encountered in trying to construct a machine which could generate the infinite improbability field needed to flip a spaceship across the mind-paralysing distances between the furthest stars, and in the end they grumpily announced that such a machine was virtually impossible.
Then, one day, a student who had been left to sweep up the lab after a particularly unsuccessful party found himself reasoning this way:
If, he thought to himself, such a machine is a virtual impossibility, then it must logically be a finite improbability. So all I have to do in order to make one is to work out exactly how improbable it is, feed that figure into the finite improbability generator, give it a fresh cup of really hot tea ... and turn it on!
He did this, and was rather startled to discover that he had managed to create the long sought after golden Infinite Improbability generator out of thin air.
It startled him even more when just after he was awarded the Galactic Institute's Prize for Extreme Cleverness he got lynched by a rampaging mob of respectable physicists who had finally realized that the one thing they really couldn't stand was a smartass.