Sri Lankan Muttiah Muralitharan is the bowler most strongly linked to the doosra. He is a freak, who has taken far more test wickets, 800, than any other bowler.
Muralitharan cannot fully straighten his arm but passed various tests aiming at verifying the legitimacy of his action.
Once the International Cricket Council discovered some of the game's finest bowlers had operated with a bent arm - as shown by retrospective biomechanical tests - it decided to settle on 15 degrees as a kind of compromise. How to judge in the moment whether an arm is bending 12, 15 or 18 degrees is near impossible.
So is the doosra legal? New Zealand Cricket's spin coach, former international Paul Wiseman, has seen shots of Muralitharan bowling it with a cast on his arm, which firms his opinion.
"It can be done legally but it is very difficult to bowl," he said.
"Murali doesn't get the same revs on the ball [with the cast]. It's very hard to bowl with many revs on it unless you're using your elbow as well.
"You've got to have a very supple wrist, put it that way, so there's very few guys who can do it legally I'd suggest."
That said, Wiseman rates it "a really exciting delivery" but he has reservations about the wisdom of teaching youngsters how to bowl it.
"If we go down that line and that kid at 15 or 17 works out how to do it and makes a World Cup under-19 side where every single bowler is put in front of a camera and he's shown to be throwing the ball, what does that do to the kid?"
Wiseman has no issues with experimentation.
"That's great, because people learn so much out of the wrist and finger position and what they can get a ball to do.
"If you're trying to bend your arm then straighten it to a certain degree then you're starting to get into a grey area that could put that kid under a bit of pressure later on, and there is pressure when someone starts talking about you doing something illegal."
Wiseman, who took 61 wickets in 25 tests, admits he's a traditionalist when it comes to bowling.
"My philosophy is you're trying to bowl with a straight arm - and if you're not then it goes away from the traditions of cricket. That's the difference between cricket and baseball."
He also pointed out offspinners don't need to master the doosra to be successful, pointing to England's Graeme Swann and Australians Nathan Lyon and, before him, Nathan Hauritz, as those who have done it the standard way but had considerable success by getting the offspin delivery to drift away from the righthand batsman. Spinners such as West Indian Sunil Narine have bamboozled batsmen with the carom ball, which comes out of the front of the hand and is flicked between the thumb and middle finger, as a fresh addition to the offie's armoury.
"We've got a number of guys around the country that are practising those. They haven't perfected it yet, and they don't spin as much as a doosra, but they're being bowled very much legally," Wiseman said.
Wiseman's former Canterbury and New Zealand teammate Shane Bond is New Zealand's bowling coach, and although speed is his thing, he finds the doosra fascinating.
His argument is more with how to police illegal actions than the actual delivery.
"That's probably the most frustrating thing," Bond said.
"There's nothing in the game that can test if a guy is legal or not. You've got to go to a static environment in a laboratory, not in the heat of battle, and until there's some technology that allows it to be tested [during a game] it's very difficult."
Bond believes it is "incredibly skilful; I just don't know if you can do it without bending your arm, so the question is whether it's 15 degrees or more. If you can that's brilliant; if you can't then you shouldn't be allowed to bowl it."
Bond has seen youngsters attempting to bowl it far more in the subcontinent, where they are more willing to experiment.
"What I know is some of our guys experiment with it purely because everybody else is doing it. That's fair enough."
West Indian spinner Shane Shillingford was suspended by the ICC after failing tests in Perth just before the New Zealand tour began.
"There's a number of bowlers around the world with a similar action [to Shillingford] so you wonder where the consistency lies. Do you test everybody?" Bond asked.
A future tip: Third umpires will have technology which will beep, courtesy of sensors, when a bowler is beyond the 15 degrees and within seconds the on-field umpire can make the call.
