Which element of handcuff design allows for adjustment according to wrist size?

The element of handcuff design that allows for adjustment according to wrist size is the locking mechanism. This feature enables the officer to secure the handcuffs snugly around the individual's wrists, accommodating different wrist sizes effectively. The mechanism generally allows the cuffs to be adjusted at various points, ensuring a proper fit that enhances both safety and security during restraint.

While other features like the chain and swivel can play roles in handcuff functionality—such as allowing for movement and preventing the cuffs from twisting— they do not contribute to the ability to adjust the fit based on wrist size. The single strand refers to the physical design aspect of the cuffs but does not influence their size adjustment capabilities. Thus, the locking mechanism stands out as the critical component that provides this necessary adjustability.