It means their handcuffs are fitted with one of these covers: http://www.handcuffwarehouse.com/cs5thmoblbox.html http://www.handcuffwarehouse.com/cs5thmoblbox.

It means their handcuffs are fitted with one of these covers: http://www.handcuffwarehouse.com/cs5thmoblbox.html http://www.handcuffwarehouse.com/cs5thmoblbox.html It covers the key holes to prevent tampering attempts, and it looks like it'd also increase the rigidity of the handcuffs (versus just the chain), further restricting movement.