Using The System Lock Key - Lenovo ThinkPad L430 User Manual

Attach a mechanical lock to the ThinkPad Mini Dock Series 3 or Mini Dock Series 3 with USB 3.0 as shown:
Attach a mechanical lock to the ThinkPad Port Replicator Series 3 or Port Replicator Series 3 with USB 3.0
as shown:

Using the system lock key

The system lock key has two positions, unlocked and locked.
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Chapter 7 Enhancing your computer