IBM THINKPAD X4 - Manual (Page 17)

If you are using the ThinkPad X40 Series computer, attach the computer,
aligning the rear side of the computer with the adjuster.
5. Press firmly on both sides of your computer until you hear a click. The
computer is now attached to the X4 Dock.
Note: Turn the security lock key clockwise to lock it. Always keep the security
lock key in a safe place. Do not lose it; without it you will be unable to
detach the computer from the X4 Dock.
Detaching the ThinkPad X4 Dock
To detach your computer from the ThinkPad X4 Dock, do the following:
1. Make sure that the battery pack is charged enough for normal operation.
About the ThinkPad X4 Dock 1-7