It's Just Basic Science.

In the video, Dillard pins Timpa to the ground with his knee in his back for more than 13 minutes. Keeping someone face-down and bound at the arms and legs is known as the "prone position," a method of restraint that is controversial in policing. Several studies have shown it may increase the risk for asphyxiation and sudden death.

Officers should know the dangers of restraining someone in a prone position, said Erik Heipt, a Seattle lawyer who specializes in cases of in-custody deaths.

“It’s just basic science: People can be essentially suffocated to death when they're lying on their stomachs in a prone position and there’s weight on their backs compressing their chest and diaphragm,” Heipt said.

“As soon as [police] have someone handcuffed, they’ve got to know to turn them on their sides and be on the lookout for any compromised breathing issues.”

For a person who is restrained and gasping for air, the instinct often is to panic and struggle.  Officers may interpret this as resisting and apply more pressure to the person’s back.

“It’s a lethal cycle that happens,” Heipt said.