Compaq Armada M300 Series Reference Manual page 68

A security selection switch on the bottom side of the mobile
expansion unit prevents the removal of the computer from the
mobile expansion unit while the system (computer and mobile
expansion unit) is docked in an Armada EM Series docking base.
To engage the lock:
1. Ensure that the computer and mobile expansion unit are
connected.
2. On the bottom side of the mobile expansion unit, slide the
security selection switch to the locked icon position.
3. Dock the computer and mobile expansion unit system in the
Armada EM Series docking base (refer to the installation
guide that comes with the docking base for docking
instructions).
To disengage the lock, eject the computer and mobile expansion
unit system from the Armada EM Series docking base. Slide the
security selection switch on the mobile expansion unit to the
unlocked icon position.