The term, "spin up," may refer to a time early in the history of IS when Canon and Nikon used gyroscopes as accelerometers. Not only did it take time to bring them to speed, but they tend to precess, rendering the image somewhat unstable in the absence of motion. There were also "inertial" sensors which vibrated like tuning forks and were sensitive to motion outside the plane of vibration. Modern accelerometers are based on inertia of a fixed mass, measured by force transducers. These sensors are inherently stable and free from precession effects.