By Joe Sterling, CNN

(CNN) -- Demonstrators have taken to the streets of St. Louis to protest the recent not-guilty verdict in the case of a former police officer accused in a fatal shooting.

Some of the protesters contend that police have used excessive force and a tactic known as "kettling" when making mass arrests. So what does kettling mean?

What is kettling?

Kettling is a term used to describe a crowd-control technique where police or security forces surround a group with a cordon of officers -- sometimes in riot gear -- and physically restrict it to a specific area.

It is used to "control access to the location and decide how to allow people to leave, perhaps through a predetermined spot," the Canadian Broadcasting Corporation, a CNN newsgathering partner, reported.

The term comes from a German word meaning cauldron.

Dick Odenthal, a retired Los Angeles County sheriff's captain, said he and his colleagues never used that specific term during his career from 1968 to 2000. But he said it appears to apply to an effort to arrest people in violent and unruly groups who refuse to obey police orders to disperse.

In some circumstances, law enforcement officers make an "arrest circle," surrounding people and giving them an opportunity to leave. If they refuse, they can be taken into custody.

"We just called it an arrest technique," he said.

Why do police use it as a tactic?

Police use kettling as a way to contain crowds who are committing crimes -- or are ignoring police orders to disperse.

CNN affiliate the St. Louis Post-Dispatch said police on Sunday moved to box in "about 100 people at a busy downtown intersection and arrest them for failing to disperse."

The police action came after "several windows were broken and concrete planters and trash cans overturned," the newspaper said.

David Klinger, a professor of criminology and criminal justice at the University of Missouri-St. Louis, said at issue is how mass arrests are carried out. Police officers understand First Amendment rights and peaceful protest, but when people engage in illegal activity, police respond, Klinger said.

A commander can order the use of force from a distance, such as the firing of a chemical irritant like tear gas -- or order a small cadre of police officers to arrest anyone who, for example, is hurling rocks or bottles at police, he said.

Police can also order those people to disperse. If they don't do so in a reasonable amount of time and the projectiles continue, police can encircle that group of protesters and arrest them. "They are defying a lawful order," Klinger said. "If you are there, you are subject to arrest."

St. Louis Metropolitan Police spokeswoman Schron Y. Jackson, when asked whether police used kettling in the disturbances, said, "Officers responded to the location where protesters gathered. The geographical layout of the area, and not a technique, dictated how tactics were deployed. "

What do opponents claim is wrong about kettling?

Some St. Louis protesters claimed that police roughed them up during the arrests and that a group was targeted for the actions of a few.

People quoted by the the Post-Dispatch cited "excessive force and chemical spray on people" who weren't among the protesters, including those who were trying to go home and members of the media.

A Post-Dispatch reporter wrote about getting roughed up in the kettle, getting knocked down, pinned and pepper-sprayed.

"We have to separate the tactic of mass arrest from inappropriate use of force when implementing that tactic," Klinger said.

Odenthal said every use of force needs to be investigated and looked at for its propriety. "If we used force, we had to justify it," he said.

Where have police used kettling?

The technique has been employed in the United States, Canada and Europe, according to news accounts.

Its use in the US came up in June, when the American Civil Liberties Union of the District of Columbia filed a lawsuit against the district and others over police behavior on Inauguration Day. Police were accused of "using excessive force, denying arrested people food, water, and access to toilets, and invasive bodily searches of protesters exercising their First Amendment rights "

The ACLU said a "small number" of protesters "caused property damage."

"In response to the vandalism, MPD officers employed a controversial crowd-control tactic known as 'kettling,' where officers corralled more than 200 protesters -- including many who had broken no laws -- by trapping and detaining them for several hours before formally arresting them," the ACLU said in a news release.

"Officers also deployed nonlethal crowd-control devices -- including pepper spray, tear gas, flash-bang grenades, concussion grenades, and smoke flares -- upon protesters and others both on the street and inside the kettle, without warning or threat of harm to officers or other members of the public."

In the suit, the plaintiffs are described as a photojournalist who covered the demonstrations and a legal observer. "Each of them suffered one or more of the constitutional, statutory, and common law violations," the ACLU said.

What should demonstrators do?

Scott Michelman, the senior staff attorney for the ACLU of the District of Columbia, said in the Washington case, police "did not have reason to believe a substantial number of the people they kettled" had committed any crime.

"They needed to go after specific people and not conduct a mass sweep," he said. "The police can't just kettle people because they think it would help control the crowd. They need to have specific probable cause."

He said laws differ state by state, and if a state has a law saying police have a right to make mass arrests of groups who've been warned to disperse, police would be in their rights to do so. "Assuming there is a law on failing to disperse and following a police order, I suspect everyone in that group would be subject to probable cause," he said.

Michelman said obeying police orders to disperse would be a smart move.

"It's generally safer and more prudent to comply with police orders and challenge the legality of those orders later," he said. The alternative is risking arrest and an escalation of the street confrontations.

The Washington Post in June quoted the police department's lead spokesman, Dustin Sternbeck, as saying that on Inauguration Day, "there were thousands of individuals who exercised their constitutional right to peacefully assemble and speak out for their cause. Unfortunately there was another group of individuals who chose to engage in criminal acts, destroying property and hurling projectiles, injuring at least six officers."

He is quoted as saying that "suspects were arrested and that 'the bulk of them are pending prosecution after being indicted by a grand jury.' He also added that "allegations of misconduct will be fully investigated."

Correction: This story has been revised to show the correct years for Dick Odenthal's career.