Most people, at one time or another, have heard about motion simulators. Usually, the phrase "motion simulator" brings to mind giant aircraft training systems used by airlines and the military. But in the past decade or so, motion simulators have started to creep in to entertainment, too. Most theme parks worth their salt have a "motion theater" or two, multi-person "pods" have popped up at fairs and promotional events, and in the past few years, motion arcades like Silicon Motorspeedway have appeared (and then disappeared), along with NASCAR chassis-on-a-platforms following the Sprint Cup circuit.
For all the recent popularity of racing simulators, though, very few people (including a surprisingly small proportion of driving simulator manufacturers) actually know how motion simulation works.
The first part of motion simulation involves your inner ear, that is, the vestibular system. For an in-depth treatment of this subject, see The Washington University School of Medicine's Auditory and Vestibular System tutorial; my goal here is to avoid using phrases like "The membranous labyrinth of the cochlea encloses the endolymph-filled scala media." To put it simply, our inner ears are filled with fluid. Our brains interpret the orientation of that fluid within our ears to decipher the rotation or acceleration we're experiencing.
The trick is that our brains, in the absence of external visual input, can't tell if we're lying on our backs, or accelerating forward at 1g. Why is it that when you lie down at night you don't suddenly feel like you've hit the throttle and dropped the clutch? Well, consider when you last took your mattress to a drag strip: context plays an important secondary role — visuals, noise, and vibration all help your brain decide whether you're in bed or approaching the chicane 30 mph too quickly.
So, a motion simulator works by replacing "real" acceleration with the force you feel from gravity:
Another way to look at it is to describe what motion simulation, and in particular our driving simulator, doesn't do: a motion simulator doesn't replicate the orientation of the car on track. A car with a soft suspension does indeed tend to roll and pitch the same way a motion simulator does, but those effects are very small. If the simulated car is driving up a hill and decelerates rapidly enough, the simulator will still pitch forward, even though the car is tilted backward, because the driver is still being pushed against his or her belts. In the same vein, a stock car on the high banking at Daytona is tilted to the left, but still cornering as hard as it can, so the motion simulator will tilt to the right to replicate the force the driver feels.
Note for the picky: Yes, our simulators do include the orientation of the car in their position. If you pull to a stop on the banking, you'll end up oriented the same way the car is—but when you're driving, this is a relatively minor part of the force you feel.