you probably don't know this but i guarantee you the cop should have learned this in training

Positional asphyxia, also known as postural asphyxia, is a form of asphyxia which occurs when someone's position prevents the person from breathing adequately. Positional asphyxia may be a factor for people who die suddenly in a hog-tie restraint by police, prison (corrections) officers, military or health care staff.[1] Positional asphyxia is also a common cause of death in infants.

prone restraint:Research has suggested that restraining a person in a face-down position is likely to cause greater restriction of breathing than restraining a person face-up.[2] Multiple cases have been associated with the hogtie or hobble prone restraint position.[3][4] Many law enforcement and health personnel are now taught to avoid restraining people face-down or to do so only for a very short period of time.[1]

Risk factors which may increase the chance of death include prolonged (particularly resisted) restraint, obesity, prior cardiac or respiratory problems, and the use of illicit drugs such as cocaine.[5] Other issues in the way the subject is restrained can also increase the risk of death, for example kneeling or otherwise placing weight on the subject and particularly any type of restraint hold around the subject's neck. Research measuring the effect of restraint positions on lung function suggests that restraint which involves bending the restrained person or placing body weight on them has more effect on their breathing than face-down positioning alone.[6]