The infinite improbability drive is a wonderful new method of crossing interstellar distances in a mere nothingth of a second, without all that tedious mucking about in hyperspace. It was discovered by lucky chance, and then developed into a governable form of propulsion by the Galactic Government's research center on Damogran.
The principle of generating small amounts of finite improbability by simply hooking the logic circuits of a Bambelweeny 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 to 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 perpetual failure they encountered while trying to construct a machine which could generate the infinite improbability field needed to flip a spaceship across the mind-paralyzing distances between the farthest stars, and at the end of the day they grumpily announced that such a machine was virtually impossible.
Then, one day, a student who had been left to sweep up after a particularly unsuccessful party found himself reasoning in this way: If, he thought to himself, such a machine is a virtual impossibility, it must have finite improbability. So all I have to do in order to make one is to work out how exactly 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 when he managed to create the long sought after golden Infinite Improbability generator. He was even more startled when just after he was awarded the Galactic Institute's Prize for Extreme Cleverness he was lynched by a rampaging mob of respectable physicists who had realized that one thing they couldn't stand was a smart-ass.
Side effects of using the Infinite Improbability Drive include temporary (and sometimes permanent,) changes to environment and morphological structure, hallucinations, and the calling into being of large marine mammals. An incredible range of highly improbable things can happen. Known effects have included the creation, and spontaneous upending, of a million-gallon vat of custard, marrying Michael Saunders, the transformation of a pair of guided nuclear missiles into a sperm whale and a bowl of petunias, redesigning the interior of the Heart of Gold, turning Ford Prefect into a penguin, transforming the desert world of Kakrafoon into an incredibly habitable oasis during a Disaster Area concert, ridding the people of Kakrafoon of their telepathy during the same concert, and allowing for the discovery of Magrathea by Zaphod Beeblebrox.