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.
Like his father, Bartolome believes in rhythm, that "the body is the engine of the swing," and in leaving well enough alone. "I do a very simple kind of work with Angel," Bartolome says.
During their last session before the 2007 U.S. Open, they worked on setup. When Cabrera is off, he crouches too much, gets too open and blocks the ball to the right. Stand up, the instructor told his friend, en espanol.
Get the right side through. "He hit a lot of three-quarter shots, trying to get synchronized," says Bartolome. "His swing seldom looks bad. He just has to have tempo."
Cabrera played great at Oakmont. When he hit a 169-yard nine-iron to mere inches at 15 on Sunday, he seemed to be in control. Then he bogeyed the next two holes, and it looked as if Furyk or Tiger Woods would catch him.
The 18th fairway is not the tightest at Oakmont, but the penalty for a short or crooked drive is severe. With perfect posture and rhythm (but with his head moving), Cabrera hit one of the greatest pressure drives in memory, a 346-yard bullet with a slight fade into the dead center of the fairway that set up a par.
A few minutes later Tiger couldn't keep it in the short grass. Cabrera won by one.
"[Cabrera's] really good, the way he thinks under pressure," Bartolome says, taking no credit for himself. "He is one guy who goes for it."
That is another thing Bartolome knows not to change.