The soft-covered PlyoCare balls are primarily meant for submaximal throwing and for constraint training, as well as general arm care. They’re not meant for throwing as hard as possible with zero mechanical intent to change; the point of PlyoCare balls is to provide a larger tactile feel in the hand that removes the connection to a baseball, further distancing the athlete from the idea that he has a baseball in his hand. Additionally, the large variation in weight between the balls creates greater changes in movement in the constraint drills (Pivot Pickoffs, Reverse Throws, Walking Windups, etc) as well, while the weighted baseballs have a much smaller weight distribution.
Here’s a short video of a comparison of two big leaguers using PlyoCare balls to subtly alter arm acceleration pathing back in the 2013 and 2014 off-season:
On the other hand, Driveline Leather Weighted Baseballs are meant for maximal intent throwing to build velocity or to affect changes at near 100% intensity for other aspects, like command or ball spin axis alignment. The black 11 oz. ball is often used for warming up purposes when it’s colder outside or to get ready in the bullpen quicker, as the heavier weight will allow the arm to lay back in external rotation easier and provide a better dynamic stretch.
Here’s a good example of how the weighted baseballs are used for our weekly velocity-development featuring Casey Weathers (Indians):
Additionally, leather weighted baseballs are used for both maximal-intent and submaximal throwing-specific work. For athletes with a good base of velocity development nearing the season, we include weighted ball command training into their programming.
Especially in cold-weather climates where warming up can be tough, the heavier weighted baseballs make good catch play substitutes, allowing an athlete to reduce the time it takes to feel loose while simultaneously giving good feedback on arm action.