Everyone knows what you mean, but the technically accurate account is that during normal use or while sitting still on a desk, the device's accelerometer regist

Everyone knows what you mean, but the technically accurate account is that during normal use or while sitting still on a desk, the device's accelerometer registers an acceleration of approximately 1 g directed towards the center of the earth, and when the phone starts falling, the acceleration drops to approximately 0. (Then the acceleration will asymptotically approach 1 g again as the device's velocity approaches terminal velocity, but well before then the software will have determined that the device is in free fall.)