"The Active Protection System (APS), available on many ThinkPad® notebook models, employs airbag-like technology to protect the hard drive. The system has an integrated motion sensor that continuously monitors the movement of the notebook, and, if a sudden change in motion is detected, it temporarily stops the hard drive to protect it from a potential crash."
The Active Protection System also supports selected secondary hard drives including:
A secondary hard disk drive installed in the Ultrabay™ Slim of your computer (T-series)
A secondary hard disk drive installed in the Ultrabay Slim of the X4 Ultrabase™ if attached to your computer
A secondary hard disk drive installed in the Ultrabay Enhanced of your computer (R-series)