Perhaps the most delicate part of the instructor's art is knowing what not to change.
The less skilled teacher tries to take away everything that doesn't look like Hogan or Tiger, an approach that in the wrong hands would have robbed the world of, for example, Jim Furyk.
Although the defending U.S. Open champion owns a swing and tempo as pretty as any, Angel Cabrera also has a decidedly nonclassic quirk.
Remember how Greg Norman used to slide his right heel all the way to his left heel on his downswing? Cabrera sometimes has a similar slide only he gets up on the toes of his right foot, a balletic move for a big man.
"Watch this," says Mariano Bartolome, Cabrera's instructor since 2001. Bartolome has isolated the most extreme example of the Cabrera cumbia, which he has cued up on a big video screen at the Jim McLean Golf School at Doral, in Miami, where he teaches.
On this swing with a driver at last year's British Open at Carnoustie, Cabrera's right foot plainly leaves the ground for a moment, toes pointed, as it travels forward toward the target on his downswing.
"The first time I noticed his right foot drag, I said to myself, Don't touch it," says Bartolome, 36, an Argentine, like Cabrera. "As long as he's on plane and his hands are square from waist to waist, he's good ... He [also] has a lot of head movement. That's good too."
Of all the instructors to the stars, Bartolome keeps the lowest profile. "I keep telling everyone who will listen that Mariano is obviously the greatest instructor nobody has heard of," says McLean. "With what he has done, any other teacher would be on the cover of international magazines."
To say nothing of what he's almost done: Andres Romero, another of Bartolome's students, finished double bogey, bogey during an amazing 10-birdie final round at Carnoustie last summer. One stroke better and he would have made the playoff, and both Open champions might have been students of the quiet man from Buenos Aires.
He was born to it, no doubt about that. Bartolome grew up in a house 50 yards from the practice tee at the Hindu Club in Buenos Aires. His grandparents ran the caddie operation, and his father, Norberto, the teaching pro, was one of the most respected instructors in South America.
Mariano was himself a very good player. He earned enough to stick on the Argentine and South American tours for parts of six years. In '93 he began his career as an instructor in earnest, spending mornings at Buenos Aires's Campo Chico Country Club and afternoons teaching the juniors at Hindu. Then, up the ladder: the Golf Club of Argentina in '96, Doral and McLean in '99.
Today Bartolome spends about half his time in Madrid, as the director of McLean's school there.