Calgary chief Crown prosecutor Gordon Wong said the Edmonton officer had little choice but to administer what is known as a "head stun" on Kristin Wilson during crowd control on Whyte Avenue on June 17.
According to witnesses, Wilson was being verbally abusive and resisting arrest on a raucous night that saw police eventually arrest close to 400 people, Wong said.
If police hadn't subdued her, they would have had to chase her, and that could have touched off a powder keg, the prosecutor suggested.
"The resulting risk of danger and the possibility that this could spark a riot meant that the police had to establish control over the complainant," Wong said in a news release.
Photos of the attack, which were shot and published by a newspaper, depict an approved takedown method, he said.
"The photographs show the officer administering a police technique known as a head stun. Officers receive training in this technique as a method of subduing someone who is actively resisting police."
The Edmonton Police Service has not commented on the case except to say none of the officers involved has been removed from duty.
Wilson, 20, of Vernon, B.C., is suing the police officer, the police service and police Chief Mike Boyd. She could not be reached immediately for comment.
In her statement of claim, Wilson alleges Const. Shane Connor struck her in the face in an unprovoked attack while she was handcuffed, causing her to fall face first into an asphalt curb.
She says she suffered a concussion, broken teeth, back trouble and psychological problems as a result. The allegations have not been proven in court.
She also claims she was not offered medical aid but instead was put on a bus, taken downtown, let off and told to find her way home.
Wilson had moved to Edmonton two weeks before the incident, which came on the night the Oilers won at home to tie the NHL Stanley Cup final against the Carolina Hurricanes at three games each.
Wilson moved back to Vernon a short time later.
The photos gave the attack wide publicity. One shot clearly showed the officer with his left arm up and Wilson's head snapping back and to the left.
Police were kept busy throughout Edmonton's playoff run. Game nights would see hundreds of Oiler fans spill out from bars on Whyte Avenue to celebrate on the south-side strip.
Some nights the parties were marred by nudity, vandalism, bonfires, drunkenness and rowdiness.
In the days after she was struck, Wilson explained she had been out with two friends that night. When they left a bar, one of the friends was pushed into a crowd and arrested, she said.
When she asked an officer where they could pick the friend up in the morning, the officer swore at her and told her to get back, she said.
She said he then arrested her, cuffed her and swore at her again as he was putting her on the bus. She swore back, his hand came up. The next thing she remembered she was on the sidewalk with two teeth slicing into her lip and blood on her clothes.
Edmonton police conducted an internal investigation, then forwarded the case to the Crown prosecutor in Calgary.