From Jayski.com:

Sucking Air"? engines at the Daytona 500? It's taken a while for word to leak out, but the big Daytona 500 trick this season - which NASCAR discovered but made no announcement about - concerned engines "sucking air" to gain horsepower. That's the word from a top NASCAR crew chief, who asked not to be named, and it was confirmed by a top car owner, who likewise asked not to be named. "When they did the chassis-dyno tests on the Daytona engines, they found that a number of teams had made huge gains in horsepower, from 18 to 25 more horsepower over the last chassis dyno tests of restrictor-plate engines," the crew chief said. "You couldn't pick up that much horsepower with a restrictor-plate engine if you worked for seven years on it. And those teams picked up that much in just one year? It's because they were sucking air. You wouldn't believe how many teams at Daytona were sucking air."
NASCAR rules are strict about sucking air - getting extra air into the engine cylinders by bypassing the restrictor plates that are designed to limit horsepower by choking down the air - and the penalties can be stiff. However no penalties were levied at Daytona. One key trick apparently was semi-legal - using a steel gasket instead of an aluminum gasket, and taking advantage of the gap of 10-thousandths-of-an-inch created when engine heat distorts the steel gasket. NASCAR officials realized they had a problem when they discovered that some of Daytona's fastest qualifiers were using engines that couldn't pass NASCAR's post-qualifying manifold leak tests. NASCAR officials were unable to confirm the legality of front-row qualifiers Jeff Burton and Jeff Gordon until six hours after qualifying had ended. In response, NASCAR quietly changed some of its rules for Talladega, and some teams that had qualified very fast at Daytona were noticeably slower.(Winston Salem Journal)(5-22-2006)