No, not necessarily g-force. Any acceleration will be measure. These devices will work anywhere even deep space with no gravity. They work because certain crystals can generate small electrical current if the crystalline structure inside are stressed (i.e. deformed). That is if you grab a crystal and squeeze it in you fingers it will generate an electrical current. Now for the way it works in an accelerometer, think back you basic physics. More specify Newton's 1st law. (paraphrased) An object at rest tends to stays at rest...unless acted upon an outside force. The crystal is mounted in an accelerometer normally at rest. Now when the device it's mounted in moves, a certain amount of force is exerted on the crystal. The crystal wants to stay still, but it's mounting points are pressing against the crystal which will deform the structure of the crystal. Just like squeezing the crystal in your fingers. And the key is that the crystals inside the accelerometer is that they only measure movement along a certain direction (aka vector in science speak). So you mount 2 (or 3) crystals so you can measure the up/down and left/right movements.