Kyle Boddy, the owner of Driveline, recently wrote an article about the differences of the leather weighted balls and the PlyoCare balls and I'm here to dumb down what that article said.
PlyoCare Balls
In essence, the soft covered rubbery PlyoCare balls are used for submaximal throwing work along with general arm care. Submaximal throwing meaning the throws you'll be making with these balls in the drills (Roll-ins, Walking Windups, Rocker throws, etc.) are not designed to be thrown at 100%. My concept of this is that they're meant for maximal effort throws, as opposed to max speed throws. The large differences in weights between the balls creates more movement in the constraint throws.
Leather Driveline Weighted Balls
These balls, as opposed to the Plyocare Balls, are meant for max intent throwing. These are used to build velocity by increasing arm strength and rotational speed with 100% intensity throws. The biggest jump in velocity can be seen while going through the Run n Gun throwing program. Run n Gun, as the name implies, is a full intensity sprint into a crow hop and throw to release the balls as hard as you possibly can. These throws are with the weighted leather balls and when charted over weeks of work can show significant changes in velocities. I'm including the Casey Weathers video again because he is demonstrating his amazing fluidity in the Run n Gun throws, which showcase the best use for the leather weighted balls.
The heavier leather weighted balls can also be used as a warm-up method. It is common for pitchers to include the 6oz weighted ball into their program throwing program to assist in warming up their arm. The 9oz and 11oz weighted balls are also commonly used to help, as they promote increased external rotation and will loosen up the arm and shoulder more efficiently. The weighted balls provide good feedback while warming up as well, if you're throwing with the 11oz ball for example and you feel an added stress it is usually because your arm path is off. A quick fix with mechanics will allow for the ball to come out smoother; teaching more efficient and less stressful mechanics in the process.
