Hibernation mode
The hibernation mode is one of three power-saving modes for the Thinkpads. In this mode, all tasks are stopped and memory data and current system status are stored on the hard-disk drive. This feature requires the presence of a hibernation file on the hard-disk drive. You can enter this mode by pressing FnF12. When in hibernation mode, all indicator lights are turned off. To resume operation, turn the power switch.
See How to make use of Power Management features to learn about getting hibernation mode and other power-saving modes to function on a Thinkpad running Linux.
External resources
Using and understanding the IBM ThinkPad advanced power management features