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 cup of hot 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 sorts of parties.
Another thing they couldn't stand was the perpetualfailure they encountered in trying to construct a machine which could generate the infinite improbability field needed to flip a spaceship across the mind-paralyizing-distances between the farthest 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 part found himself reasoning this way:
If, he thought to himself, such a machine is a virtual impossibility, then it must logically have 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 the one thing they really couldn't stand was a smart-ass.
-Douglas N. Adams