The Infinite Improbability Drive simultaneously places a starship at every conceivable point in the universe, thereby eliminating all that tedious mucking about in hyperspace. It uses vast amounts of improbability, and many silly things happen in its wake.
The measurement of improbability is as follows (at least in the H2G2):
The higher the improbably the faster the spaceship travels. When the improbability equation is 1 to 1 or nominal then the ship is stopped.