The FitBit is a piece of wearable technology that uses a three-axis accelerometer in order to try and understand and dissect a user's movement. The accelerometer has special technology that transforms the body movement into a data point it can use to give a readout on the number of steps taken even when running.
Because FitBit uses a three-axis accelerometer, the device can measure movements of the body, no matter which direction the body moves. The algorithm in the device is also geared to read data such as patterns of movement, duration and intensity. By taking the data from all these movements, the device can tell when someone is running as fast as she can, when she is walking, and when she is moving in a vehicle and not actually taking any steps.
The FitBit is able to recognize when a motion isn't big enough to be a step. As with most devices that work using algorithms similar to the FitBit, the device can be fooled if there is something that causes movement that looks like someone is taking a step when she isn't. Riding in a car on a bumpy road is one kind of motion that can make the device read the bumping as steps.