Police detectives in Manassas, stymied in their investigation of the hit-and-run death of a 10-year-old Vietnamese girl, have turned to a Maryland hypnotist to try to get leads in the case.

"It was just like listening to someone watching TV," said Detective John Collier, who attended the hyponosis sessions with three witnesses to the killing. "It was just like they had it on videotape and they were playing it back in slow motion."

As a result, police say they now have a good description of one of two vehicles that struck the girl as she crossed the street after nightfall Feb. 7.

Such hypnosis sessions are controversial -- critics call them mere gimmickry -- and formerly appeared to be banned in Virginia by a state law. They may soon become more common inside the state because Gov. John N. Dalton on Tuesday signed a law repealing all restrictions on hypnosis.

Those restrictions forced the Prince William County police to drive the witnesses 50 miles to the office of hypnotist Richard Ittner in Columbia, Md.

During the past two years, police in Northern Virgina -- including Fairfax County as well as Price william -- have taken more than a dozen witnesses and victims of crime to undergo hypnotism in Ittner's Maryland office, where the procedure is unquestionedably legal.

The bill lifting the restrictions on hypnosis passed the Virginia House of Delegates in February by a 94-to-2 vote, and received a 40-to-0 vote in the state Senate. When it goes into effect July 1, Virginia will join Maryland, the District of Columbia and most other states that have no laws restricting hypnotist, according to the Association of Advanced Ethical Hypnosis, a national group with 1,500 members.

Under the old laws, hypnosis could be used in Virginia only by physicians, dentists or clinical psychologists "in the practice of [their] profession." Though violation of the law carries a gine of $500, Del. Theodore V. Morrison Jr. (D-Newport News), who introduced the repeal measure, said he could find no record of anyone ever having been charged with such a crime.

Morrison said he introduced his bill after the Virginia State Police asked that police investigations be exempt from the restrictions. "We just weren't sure if we could take a witness to be hypnotized in Virginia," said Jay Cochran, director of the criminal investigation bureau of the state police. "Rather than take a gamble on it, we asked for a change in the law."

Cochran said that the state has no plans to train its own officers to conduct hypnosis, as police departments have done during the past three years in Montgomery County and elsewhere.

Cochran and other officers see hypnosis as a definite plus for police. One example cited by hypnosis advocates is the 1976 school bus kidnapping case in Chowchilla, Calif. In that case the FBI brought in a hypnotist who got a witness to recall a license plate number.

Even though there soon will be no restrictions on hypnosis in Virginia, some prosecutors view its use in criminal cases with considerable skepticism.

"It's a gimmick," said Fairfax Commonwealth's Attorney Robert F. Horan Jr., "and I hate to see criminal investigations get bogged down in gimmicks . . . It's done extensively in other parts of the country. In California it's a big deal. But it's so open to suggestion that it's not the best evidence in a trial. I've told detectives not to use it unless they've absolutely reached the pits of an investigation and have nothing else to turn to at all."

Even some who practice hypnosis have their doubts about how much it can be used in criminal cases.

"What is obtained under hypnosis is not necessarily the truth," said Melvin Gravitz, a clinical psychologist in Washington who is past president of the American Society of Clinical Hypnosis, a group that includes 3,700 physicians, psychologists and denists."It has to be checked on and investigated properly with due regard for the law."

Gravitz, who has performed hypnosis for the FBI and D.C. Police, said the technique often can be useful in providing investigative leads.

Gravitz added that he believed the Virginia action repealing all restrictions on hypnosis is unwise.

"Anyone conducting hypnosis should be well grounded in basic human behavior," he said. "They might encounter emotional situations that they can't control. When you free up situations through hypnosis, you can come across some very distraught individuals."

Ittner, the hypnotist used by Fairfax and Price William police, is a 38-year-old Presbyterian minister with training in pastoral counseling and hypnosis. He is not a licensed clinical psychologist, but he said he has a private practice as a psychotherapist.

In the past two years, Ittner said he has conducted hypnosis to sharpen the memory of witnesses or victums in about 90 police cases in the Baltimore area. Montgomery County and Northern Virginia.

Fairfax police spokesman Warren Carmichael said that as a result of interviews with Ittner, police have been able to prepare a composite drawing of a suspect in two rape cases in Reston, but so far no arrest has been made.

carmichael said police were unable to get any leads in three homicide cases before Ittner interviewed witnesses.

In Prince William County Detective Collier said that under hypnosis three witnesses agreed that the van that struck the girl in Manassas was "cocoa brown with a Chevy emblem on the front of the grill with a lot of chrome." The van had a cut-out window, he said in the shape of a mushroom or square.

The hypnosis sessions are not like "the TV stuff you see when it's done someone is asleep," Collier said. "The people are just very relaxed, which allows them to recall things they might not otherwise."