What feature of handcuffs prevents the single strand from moving?

The feature of handcuffs that prevents the single strand from moving is the double lock. When applied, the double lock mechanism secures the ratchet in place, ensuring that the cuff remains at the designated tightness and cannot be further tightened by the ratchet or expanded accidentally. This is crucial for the safety of both the detained individual and the officer, as it prevents potential injury or escape by maintaining a secure fit without excessive pressure. The double lock is an important safety feature that provides additional control during the use of handcuffs in various law enforcement situations.

In contrast, the other options serve different functions: the key actuator is used to unlock and adjust the handcuffs, the chain typically connects two cuffs to allow for some movement of the wrists, and the swivel allows cuffs to rotate, which can help reduce the risk of injury to the wrist. Each of these elements contributes to the overall design and functionality of handcuffs, but it is the double lock that specifically addresses the issue of preventing any further movement of the cuff once it is applied.