Except, there's really no such thing.

"The studies do not support the contention that ventilation is significantly comprised, regardless of being placed prone and restrained; being restrained hogtied or placed in a prone position, with or without weight on the back; or restrained, hogtied, and exposed to pepper spray. From a medical perspective, these studies reveal that there is no evidence that the hogtied position, in and of itself plays a risk factor for asphyxia. Collectively, the studies conclude that the prone restraint position, nor the hogtied restrained position, with or without weight force, contributed to abnormal pulmonary function, hypoxemia (deficient oxygenation of the blood), hypoventilation (decreased lung ventilation), or hypercapnia (excessive amounts of carbon dioxide in the blood) sufficient to cause asphyxia. Moreover, the findings are supported by prior work on exercise physiology which indicates that arterial oxygenation improves rather than decreases with moderate exercise in healthy people (Levitzy, 1995; Wasserman, et al. 1994)."

"The physiological science of restraint not only clarifies that the restraint position or restraint procedures fail to support a sudden death from ventilatory compromise but also reveal that other more significant factors are more likely to contribute to a sudden violent restraint death. Other factors such as excited delirium, drug intoxication, mental illness, stress, trauma, and catecholamine hyperstimulation, are considered to be factors playing the most significant role in these sudden deaths (Ross, 1998; Ruttenber, et al., 1997; Wetli, 2006, 2005; Laposata, 1993; Karch, 2008; DiMaio and DiMaio, 2006). Moreover, other studies show that a significant number of individuals who suddenly die after restraint have an abnormally enlarged heart, linked to chronic drug abuse, and have internal organ deficiencies (Herd, 1991; Karch, 2008; Karch and Green, 1995; Karch and Stevens, 1999; Laposata, 1991, 2006)."

Long, but worth the read