It’s well known that Bob Feller brought home a military-grade gun scope from World War II, which his team used to pinch signs. Less known is why he was so passionate about the topic.

Feller was certain that he’d been beaten before by teams with similar schemes, specifically the Tigers. These feelings were spurred in part by a 1940 game in which Detroit torched him.

As it turned out there was a scope in the Tigers’ clubhouse that August, belonging to pitcher Tommy Bridges, a recently bought hunting tool he was eager to show off to his teammates. As soon as the lens was displayed, however, it didn’t take long for a secondary use to surface. “Someone suggested that we equip one of our bullpen pitchers … to zero in on the catcher’s signs,” said Hank Greenberg, the Hall of Fame first baseman, in The Complete History of the Home Run. “Then, by a hand signal, the catcher’s sign could be relayed directly to the batter.”

The spotter, stationed in the Tigers’ center-field bullpen, relayed his information to a team member leaning against the wall. For fastballs, the player rested his hand rested on the fence; for curveballs he did not.

That September, the Tigers offense sprung to life like never before, surging into the league lead in batting average and on-base percentage, finishing second in slugging and third in home runs. When Yankees manager Joe McCarthy became suspicious of uniformed Tigers personnel bearing binoculars, the team shifted its spy—usually a starting pitcher whose turn in the rotation had just passed—to the bleachers, where he sat in street clothes and signaled from the crowd.

In third place, four games behind Cleveland on Sept. 3, the Tigers—losers of 15 of their previous 23—went on an 18-7 run to close the season and take the pennant by a single game over the Indians. All but three of their final 25 games were at home. Said Greenberg, who won that season’s AL MVP Award, “I never had a more enjoyable month.”