Most of modern harddrives have an autopark function. Old drives had to be parked before shutting them off. It's like with a record player: you have to put the pickup arm beside in advance to remove the record iteself. A harddrive has pickup arms, too, and they have to put away before the disk stops. The reason for this: The pickup arms are flying above the disk with the wind the rotating disk produces. When the disk stops, the pickup arm will land on the surface and scatch it and destroy the data written on the disk.

Autopark is a little energy buffer that rips the pickup arms into the innermost part of the disk where they can land without damaging active data.