Gordon Wong of the Calgary Crown's office said Kristin Wilson of Vernon, B.C., was at the receiving end of an approved police technique called a head stun when she fell to the pavement and smashed her face.
But despite repeated questions from reporters, Wong wouldn't reveal what a head stun was.
He also wouldn't say how much force should normally be used, where on the head the force should be delivered or how prosecutors determined it wasn't excessive.
But he did note "it was not a slap to the head that knocked her to the ground. There was actually a deliberate move to take that person to the ground."
Wong said that according to witnesses, Wilson was verbally abusive and resisting arrest on a raucous June 17 night on Whyte Avenue - the city's southside party strip - when the Oilers won Game 6 of the NHL Stanley Cup final against the Carolina Hurricanes to tie the series.
Police eventually arrested close to 400 people that night.
Wong said if police hadn't subdued Wilson, they would have had to chase her and that could have touched off a powder keg.
"You have to understand the volatile nature of what was going on," said Wong. "Within the week before you had active rioting taking place on that street - bonfires."
Tom Engel of the Criminal Trial Lawyers Association policing committee said he was shocked by the decision.
"I don't care what she did beforehand. The fact of the matter is she was cuffed, she was fairly stationary and to use a head stun is grossly excessive.
"If she's trying to run away, grab her. You don't punch her as hard as you can in the head. That's what a head stun is. They're trained to punch as hard as they can, aiming for the temple area."
Engel said there was enough evidence to support an assault charge and wondered whether Wong and his staff had done their homework.
"How come he doesn't know what a head stun is? I really question the quality of review by the Calgary Crown's office."
The office was asked to review and give a recommendation on the Edmonton Police Service's internal investigation of the matter.
Staff Sgt. Greg Alcorn said it is now up to Chief Mike Boyd whether to lay charges.
Alcorn said the force is sensitive to the public's perception of the photos, one of which clearly shows the officer with his left arm up and Wilson's head snapping back and to the left.
"We obviously want to ensure that the public have confidence in us at all times," said Alcorn.
None of the officers involved has been removed from duty.
Wilson, 20, is suing the police officer, the police service and 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 encounter and moved back to Vernon a short time later.