Six weeks ago, Clayton Kershaw went looking for his slider. He had just retired the first three Diamondbacks in order, but the breaking ball felt wrong. So he loped down the steps of the Dodgers’ dugout, turned right into the tunnel and faced the wall that players use for target practice—throwing baseballs when they are satisfied with their performance and stray equipment when they are not.
In his left hand, Kershaw cradled a blue weighted “plyo ball.” These training tools have become popular throughout the sport over the past few years, but until recently Kershaw disdained them. This one weighed 450 grams, more than three times as much as a standard MLB ball.