From the LWN kernel page discussing implementing support for the IBM “hard drive active protection system”:
The theory of operation here is that a user-space daemon will be monitoring the status of the system, as reported by the accelerometer. Should this daemon note that the laptop has begun to accelerate, it will quickly write a value to the protect attribute for each drive in the system. The drives will respond by parking the disk heads, and, in any other possible way, telling the drive to crawl into its shell and prepare for impact. Once the event has transpired, the shattered remains of the laptop can attempt to resume normal operation.
Thanks for the laugh John!