Mitchell Johnson reignited the Ashes with a breathtaking display of bowling in the third Test in Perth, taking nine wickets as Australia squared the series at 1-1 with two games to play.

His play also represented one of the most remarkable individual turnarounds in recent times. Wicketless in Brisbane, dropped in Adelaide, Mr. Johnson had so spectacularly failed to live up to his fearsome reputation in the opening two games of the Ashes that he'd been dubbed Myth Johnson by some fans.

Australia's Mitchell Johnson, center, appeals successfully for the wicket of England's Paul Collingwood—one of nine wickets Mr. Johnson took during the third test. Associated Press

But here, finally, was an appearance by the real Mr. Johnson, the one that slings down 90 miles-per-hour thunderbolts and was named cricket's world player of the year in 2009.

His spell on the second day in Perth, in which he took four wickets in 27 balls and consistently conjured devastating late swing, could prove to be the moment that swung the Ashes. It was also as devastating a spell of fast bowling as has been seen in 133 years of this ancient rivalry.

"All things considered, it must go down as one of the finest Ashes spells ever," said Scyld Berry, the editor of the Wisden Cricketers' Almanack, the annual bible of the cricketing world. "Great pace, great swing. And crucially England did not give it away. He took it from them."

Just don't ask Mr. Johnson to explain how it happened.

Here's one of the most persistent quirks of cricket: Swing bowling—the black magic art that gets the ball boomeranging through the air—is arguably the most prized attribute in the modern game. But even the best swing bowlers don't always know what they're doing.

"My role is of a strike bowler," Mr. Johnson said before the series. "I hit the deck hard and if the ball swings, it swings."

This much we do know: When the ball starts to swing, it doesn't matter how flat the pitch, how big the bat or how great the reputation, even the best batsman can be be made to resemble a man in a blindfold.

Even under good conditions, a batsman has only a few fractions of a second to decide how to cope with a ball delivered from 22 yards at speeds in excess of 85 miles-per-hour. When it starts deviating unpredictably—and particularly when that movement occurs late in the ball's trajectory—batting becomes next to impossible.

Mr. Johnson also added 63 runs with his bat during the third Test. Associated Press

"I don't think any batsman enjoys facing good swing bowling," said Simon Jones, the former England bowler. "At 85mph-plus, it's very difficult."

But when it comes to swing bowling, the batsman isn't the only one wracked by uncertainty. Even the best practitioners of swing are often oblivious to how or why they are able to make the ball move so prodigiously on one occasion, yet at other times nothing happens, the ball refuses to budge off a straight line and the batsmen prosper.

"You can have a checklist of 10 things you need to get the ball swinging and then some days you can tick nine out of 10 and it goes as straight as a railway line," said Jacques Kallis, the South Africa all-rounder. "Other days you can tick four and it swings."

The condition of the ball is one factor. Most observers believe the weather also plays a major role, with overhead cloud cover considered conducive to swing bowling. In the case of Mr. Johnson, biomechanics are also at work. Having seen his bowling carted around the park in the first Test in Brisbane, Mr. Johnson spent a fortnight working with Troy Cooley, the Australian bowling coach, on his body shape during his run-up to the stumps. "Not a lot has changed technically, it was just his position at the crease," said Mr. Cooley.

Whatever the reason, when the ball is swinging late, it is lethal for batsmen. And when the ball is swinging late for a left-handed fast bowler like Mr. Johnson, it can be almost unplayable.

Deliveries don't come more difficult than from a left-hander slinging down 90-mph fizzers with devastating late swing. The reason for that is simple: Unlike right-arm swing bowlers, who move the ball away from the batsman and can regularly beat the bat without drawing the vital edge, the left-armer has no such hindrance. If the ball is fast and full, the batsman has two choices: To be bowled or to given out leg-before-wicket.

"When he's swinging the ball late, he's a world-class bowler," said Damien Fleming, the former Australia batsman. "And his pace is a concern for the batsmen as well because you don't want to get hit. Whatever he worked on with Troy Cooley, we've seen the results."

All of which is bad news for England ahead of the fourth Test, which begins in Melbourne on Sunday.

"This spell has transformed him and transformed how they think about him in the English dressing room," said Ricky Ponting, the Australian captain. "Some of the deliveries he bowled will have them seriously thinking about how they play him."

Write to Jonathan Clegg at