This idea owes its existance to new baker [corezz]'s new idea "Moving Day", which I have linked below.
There are phones and other game-capable platforms which use accelerometers as, or as part of, the user interface to the game.
However, if you were to try to play such a game while riding a bus,
or train, or whatnot - any starts, stops, turns, or bumps get applied directly to your gameplay.
Differential accelerometry uses accelerometers attached directly to the vehicle itself; data about the experienced accelerations is then broadcast via a short range WiFi link and is picked up by your hand-held device, which can then compensate.