Such restraints are often used in the United States in courtrooms, or for transporting prisoners, or in other public situations as a safeguard against escape.
Belly chains are used because there still remains a relatively large freedom of movement to the detainee when their hands are cuffed in front of the body.
As an alternative, the hands could be cuffed behind the detainee's back, but this will soon inflict discomfort and even pain when being handcuffed like this for a longer period of time.
These restraint belts have a metal ring on the front, through which the handcuffs are plugged and then put on the detainee's wrists.
The belt is then placed around the detainee's waist and secured with a buckle; some models can also be locked with a padlock.