Valenzuela learned the pitch two years earlier from Bobby Castillo, a reliever in the midst of an otherwise-forgettable career. “It took me a while,” Valenzuela said. “But it ended up being my best pitch.” That season he won his first eight decisions and ultimately became the only rookie to win a Cy Young Award. Valenzuela’s success for most of the ’80s helped keep the screwball on the map. Willie Hernandez’s M.V.P. season followed. So did a perfect game in 1988 by the occasional screwballer Tom Browning of Cincinnati. You could spot the pitch well into this century, but over time it died out. Four months into the season, it’s safe to say that Santiago is the only pitcher who has thrown the screwball in a game this year. “I can’t remember the last time I saw one,” says Tim McCarver, the former catcher and a longtime announcer.

The screwball’s decline can be attributed partly to the emergence of other deceptive off-speed pitches: the circle change, the cutter, the split-finger. (Though not one of these is a serviceable replacement for the screwball, which enables pitchers to throw a ball that breaks away from opposite-handed hitters.) A complete explanation is more complicated, if not entirely logical. The late ’60s and early ’70s, when half the teams in baseball might have had a screwballer on their staffs, was the era of squeeze bunts, of hit-and-runs, of lollipop curves meant to entice ground balls. A preponderance of games ended in scores like 4-3 or 3-2. Today, by contrast, big-league lineups are packed with players who can hit home runs, and nearly every pitcher looks to deny them with strikeouts. In this battle of heavy artillery, there’s no room for the cavalry. “Power has become the name of the game,” says Alan Dunn, the pitching coach at L.S.U., which had four pitchers chosen in last year’s amateur draft. “You’re looking for dudes who are going to come in throwing 96, 97, and just power their way through.”

A screwball can make a hitter look silly, but it is not especially a strikeout pitch. With its unpredictable downward break, it yields ground balls. “You get a lot of easy, ugly outs,” says Mark Gubicza, who spent 13 seasons throwing sliders for the Royals. “The ball starts right there, you think you’ve got it, but it swerves, and you just get a piece of it. That’s not power. But it’s effective.”

In a power culture, the screwball’s potency has been forgotten. Strikeout pitchers usually get the scholarship offers and wind up in pro ball. It’s only when a pitcher fails to become the next Nolan Ryan that he starts looking for ways to keep a roster spot. He might try throwing sidearm, adding a cut fastball, even a knuckleball. If an area of opportunity for the screwball exists in today’s game, it is in these interstices between success and failure. “You need to find somebody in the minors who has all the intangibles, a great kid, just a little short on talent, not quite going to make it,” says Rick Waits, the Mariners’ pitching coach. “And you go to him and say: ‘You need one more pitch. Just an edge. And here it is.’ ”

“That,” Hector Santiago said when I related Waits’s explanation, “describes me to a T.”

Without the screwball, according to Joe Moeller, an advance scout with the Miami Marlins, Santiago is “a below-average big-league pitcher.” I was sitting with Moeller and other scouts one March afternoon, watching Santiago throw. Moeller seemed surprised that someone with a pretty good fastball, spotty control and not much else would begin the season in a contending team’s starting rotation.

Then he saw Santiago break off a pitch that kicked so severely that the batter, Logan Schafer, asked the catcher, Hank Conger, what it was. Moeller was equally impressed. He had not seen Santiago’s screwball before, and he nearly jumped from his seat. “If he can throw it like that,” he said, “that’s definitely what we call a ‘plus pitch.’ Put that in the mix — now he’s compelling.”