I can tell you right now what Pressly is doing wrong. He's opening up his chest too early, especially on his breaking pitches. That causes the ball to corkscrew, like it's on a barbecue skewer, rather than tumbling forward (topspin), which would make the ball dive. A good slider has a blend of corkscrew and tumble action, just enough tumble to make it break downward, about the same time it breaks left. Especially on his two hanging sliders for home runs, Jack Morris noted that they did not drop at all because they had "washing machine" spin, which is corkscrew. Now, the difference between a curve ball and a slider essentially is velocity. A slider is supposed to arrive at around 87 to 89 mph, where a curve ball is usually low 80's, all the way down to the 50's. A pitcher doesn't try to create spin on a slider by passing his hand around the outside of the ball, but rather by the grip, a wrist flick, and body position at release. Otherwise his fingers are behind the ball because he plans to throw it hard. Pressly has been squaring up his chest to the plate on his slider too early, removing the forward tumble from his slider. The pitching coach should tell him to throw his slider from a more shoulder-closed release point, the way Bert Blyleven used to do. Old Bert never worried much about making pitches dive, right? That's why. Just throw that slider with more of a closed front shoulder. Then it will dive just fine.