That much is undisputed; what followed next is less clear. Kingsley testified that a sergeant kneed him in the back, and that two officers then slammed his head into a concrete bunk. This caused him pain so severe that he couldn't immediately walk again. The jail officers, by contrast, said that Kingsley was resisting their efforts to remove his handcuffs. At this point Kingsley was handcuffed and lying face down. Both parties agreed that the officers finally ended the altercation by applying a taser to Kingsley’s back for around five seconds. Kingsley recalled an officer saying, “Taze his ass!” just before the electric shock.

The Eighth Amendment, which prohibits “cruel and unusual punishment,” only applies to prisoners who have already been convicted of a crime. The Fourth Amendment is generally interpreted to protect against unreasonable police actions against people prior to their arrest. Before Kingsley, people who had been arrested and detained, but not yet convicted, fell into a legal void.

To fill in the constitutional gap, Kingsley argued at trial that the officers’ use of excessive force violated his civil rights under the 14th Amendment’s Due Process Clause—one of the cardinal provisions of the Constitution. It forbids any state actor, such as a police officer, from depriving a person of “life, liberty or property without due process of law.”

The case turned on what instructions should have been given to the jury at trial level. Kingsley argued that the words “reckless” and “reasonable belief” had been repeatedly and improperly used in the jury instructions. The Court agreed. During oral argument, Justice Sonia Sotomayor observed that police officers seemed to want the authority to “get a free kick” when managing pretrial detainees. The Court’s decision makes it harder for police and corrections officers to get that constitutional free kick.

Kingsley marks a departure from a traditionally deferential posture to police testimony both inside and outside courtrooms. In the 1985 case of Tennessee v. Garner, the Court ruled that so long as police officers have a reasonable belief that his life or others are in danger, their use of force will likely be proportionate in light of the government’s interest in effective law enforcement. The Eighth Amendment bars only correctional force applied with “deliberate indifference,” or exercised “maliciously” or “sadistically” against inmates.

The less deferential holding in Kingsley supports research showing that ordinary citizens hold very different views than the police on the perception and tolerance of abuse of force. A recent study of 3,230 sworn officers in 30 American police departments showed that new or novice officers were more likely to view excessive force as worthy of severe discipline than officers with a moderate level of experience. These newer officers were also more likely to report such incidents than their more experienced peers. This suggests a gap between the public and police perceptions of excessive force, a gap that widens with exposure to institutional culture.