The dummies, which stand in the batter's box as imitation hitters, were introduced to help pitchers practice throwing over the inside part of the plate. The purpose of the string is similar in concept, though it is used to emphasize the importance of pitching over the lower part of home plate.
"It's just a focus point for your eyes," Kerrigan explained. "You see it in your pre-set routine. You're looking down at it. It's a teaching aid."
Here's how it works: Kerrigan has tied pink landscaping string to vertical pipes on each side of a row of four home plates, which are across from the mound where pitchers line up to throw their side sessions at Pirate City. The height of the string is about a ball's width lower than Kerrigan's knee, meaning that it loosely frames what would be just below the strike zone.
The goal isn't to hit the string. But when pitchers train their eye on it as they prepare to deliver a pitch, the result is often that pitch cross the plate lower. That's precisely the goal.
"It's a good reference point to let you know where you are in the strike zone when the ball crosses the plate," starter Charlie Morton said of the string. "When that line is there, you can see whether you're above or below. He's definitely emphasizing keeping the ball down, especially this early in the year, when guys are just starting to throw off the mound again."
Kerrigan said he also plans to bring the string over to McKechnie Field in March when the Pirates shift their activities to that facility.
As for the Pirates' two dummies, they have not made an appearance during the Spring Training bullpen sessions yet. However, Kerrigan promised they would be brought out soon.
The second-year Pittsburgh pitching coach has another teaching tool in his bag of tricks as well. Each catcher has a strip of rubber (approximately the width of one-third of home plate) that can be moved from one side of the plate to the other to dictate which corner the pitcher should be throwing to. Again, it is used as a target for the eye.
"I haven't seen that before," Morton said of the strips. "I think it's good, too. It gives you a focal point. Plus you can move it around. That way you can see the angle of the breaking ball and where it's supposed to go."
For instance, the strip is placed completely perpendicular to the back of the plate when a pitcher throws a fastball because the track of the ball is straight over the plate. For a curveball or breaking ball, though, the strip is angled across the plate as a visual for the path that the pitch should follow.
Jenifer Langosch is a reporter for MLB.com. This story was not subject to the approval of Major League Baseball or its clubs.