Mattingly is an unlikely design maverick. During his time as a Yankee he was known for his work ethic and his commitment to the game and not his endorsement-chasing. He retired in 1995 after 13 full seasons in the big leagues, all played in the Bronx. He returned to his home in Evansville, Indiana, and like many middle-aged parents, spent a lot of time at the local baseball diamond watching his three boys play little league. Over time, he realized that many of the kids, including his own, were gripping the bat stiffly in the palm of their hands instead of in their fingertips. "The bat is not meant to be swung with muscle," he says. "It's meant to be swung with leverage and quickness and snap."

During his playing days, Mattingly shaved his handles to create a custom fit; it occurred to him that a triangle-shaped handle would make it easier for his kids to grip the bat correctly. The prototype was a success, and he realized that he had inadvertently stumbled on an innovation. "Basically the grip puts the bat in your hands the proper way," he says. "It really relaxes your hands and then it relaxes your arms." As a result, a player is able to whip the bat through the strike zone faster, adding yards to hits. In testing by Mattingly's company, kids using a "V-Grip" bat increased their bat head speed by about 7 percent on average, which adds an additional 18 feet on drives. 

"The grip really sets them apart," says Kyle McDaniel, product manager for online retailer Justbats.com.

Mattingly, with help from Jim Wells, a hardwood lumber inspector, spent about a year further developing the handle and then patented the design. But he had trouble figuring out the next step.

"I didn't really have a business background," said Mattingly, who entered the Yankees farm system right out of high school. "I wasn't out there in the world working on, How do you get this to the marketplace?" On a flight from New York to Cincinnati he found himself next to Yankees fan and businessman Skip Shaw, who had worked at Sikorsky Aircraft and was then at Accenture. "I didn't recognize him when he sat down next to me," said Shaw. But after exchanging a few pleasantries, he realized: "Oh my God, I'm sitting next to Donnie Baseball."

By the end of the flight they were friends, and less than a year later Shaw had become president and C.E.O. of the company. Mattingly, who was just hired as the batting coach for the Los Angeles Dodgers, is still deeply involved and attends every board meeting. But, he admits, "I'm not sitting here 24/7 in the office." He is updated via email almost daily, and, in addition to helping market the bats, approves every new product.

Soon that may include a lot more than just bats, which range in price from $35 for a T-ball bat to $350 for an adult composite bat. From the beginning, Shaw says, he didn't want to be "a little niche bat company, but to really compete with the other major brands in the sporting goods equipment business." The company is currently looking for game-changing innovations in a number of different sports.

At first, JustBats.com had concerns that the bats wouldn't sell because of their odd shape. But the V-grips have developed a loyal following and a "very solid return-customer base," McDaniel says. "Kind of contagious." To help customers understand the design, Justbats.com will soon have an online demonstration video. But McDaniel admits Mattingly "has a long way to go to compete against Louisville Slugger and Easton."

While Mattingly Sports makes bats for every level of play, the company is focusing on the youngest players who are the most willing to change their swing. "We want everybody to use it, but I'm after those kids in little league. I'm after those kids in T-ball," says Mattingly. The company hasn't yet bothered to get its bats approved by Major League Baseball. "We know we have to build from the bottom and not start at the top."