There are a few reasons why it’s nearly impossible to throw a curveball in Denver. The dry air makes it difficult for a pitcher to grip the seams on the ball and to get the friction between the seams and fingertips necessary to make the ball snap as it leaves his hand. It also diminishes what break the pitcher is able to put on the ball, turning even the best curveball into a flat, rolling batting practice offering. All this also affects the way Rockies pitchers throw on the road, as pitching in Coors Field takes the curve out of their arsenal, leaving them out of practice in throwing the pitch properly and unwilling or unable to throw it in crucial situations on the road.

If anyone’s looking for one key reason why Shawn Chacon has become an excellent pitcher since joining the Yankees, this is it. The man throws two different curveballs – one of them a sharp little twister that comes in on the hands of right-handed hitters, the other a big bender he can throw to either side of the plate or down the middle – and effectively changes speeds on all of them. These pitches make the high fastball he goes to in strikeout counts look harder than it is and induce plenty of pop-ups.

With the curve working last night and some uncharacteristically excellent defense from Jorge Posada, who threw out the speedy Chone Figgins at second in the fourth and then took advantage of a wild pitch that was luckily deadened by the home plate umpire’s ankle to throw out Vladimir Guerrero at second two batters later, Chacon looked unbeatable into the sixth inning of last night’s game against the Angels.

Unfortunately for the Yankees, so did John Lackey, who pitches pretty much the same way Chacon does – using a couple of different curveballs, the speed and break of which he’s able to vary effectively and which he can locate at different spots in the strike zone, to set up an adequate fastball he’s unafraid to throw over the heart of the plate when a batter’s not expecting it.

Still more unfortunately for the Yankees, it was Chacon who wore out first. The back-to-back run-scoring doubles he gave up to Figgins and Orlando Cabrera in the sixth came on curves that had lost just enough of their bite and velocity to quicken the hitters’ reaction times. In different circumstances, with a different bullpen – the one Joe Torre had last night was weak enough that he was forced to uncharacteristically get set-up man Tom Gordon warming up to prepare to pitch the seventh – the Yankee skipper would probably have gone to a reliever, as Angels manager Mike Scioscia did when Lackey looked to have similarly lost his edge in the bottom of the sixth.

Right in that inning, you saw all the Yankees’ strengths and weaknesses. The strengths? A magnificent offense – Alex Rodriguez worked a walk, moved to second on a grounder, and scored on a hard-hit single by Gary Sheffield. This group scores runs accidentally. Then there was Chacon, the cast-off from the Colorado Rockies of all teams, who managed to get out of the inning without giving the game away, even struggling with visibly diminished stuff and facing Vladimir Guerrero with a man on.

The weaknesses? A disproportionate reliance on a few hitters and a staggeringly thin bullpen that left a clearly spent Chacon having to come out to start the seventh and an equally (though for different reasons) spent Al Leiter to relieve him.

None of this mattered in the end; Leiter got the double play the team needed and got out of trouble. A bad throw home by Figgins on a Derek Jeter grounder, a big hit from – of all people – Ruben Sierra, and a typically effortless performance from Mariano Rivera, and the Yankees were set to move on to a decisive Game 5. Chacon, Leiter, Robinson Cano; it just wouldn’t have fit had anyone else conspired with Jeter and Rivera to, once again, save the Yankees’ season.

Still, it’s hard not to wonder when the bill’s going to come due. Last night’s game certainly had a sense of doom about it; every Yankee comeback and every nice play had a bit of luck or unlikely Angels failure to it, and this was certainly one of those games where one or two clearly identifiable bounces or breaks going the other way would have led to a different outcome, and a harsh verdict on how the Yankees’ failures came back to haunt them. There’s still time for that verdict to be passed, but for now they’ve come through again, and once again there’s no mystery as to why.

tmarchman@nysun.com