Canon CLC5151 Reference Manual page 26

In Order to Avoid Unauthorized Use of the Machine
Unauthorized copies can be prevented by using the optional Key Switch Unit to manage the operation of
the CLC5151/CLC4040/iR C4580i/iR C4080i. The use of this key should be strictly supervised.
xxiv
Security Key
When using the machine, insert the
security key into the main unit, and
then turn it to the right.