This is latest entry in my Texas Rangers Prospects video scouting series. It features LHP prospects Martin Perez of whom you've heard good things and Juan Grullon of whom you've probably heard nothing.
Perez pronates like this on each of his pitches - fastball, curveball, and change up - and it is excellent for both health and performance. Pronation allows for better ball rotation (more spin therefore more movement) and helps protect the elbow from forearm flyout. This is a good thing because Perez doesn't get his arm nearly as vertical when he throws his fastball and change up compared to when he throws his curveball.
The simplicity of his motion allows Grullon supreme control over his follow-through. Under such control, Grullon's pitching arm stays on the pitching arm side of his torso; it does not wrap across his body. His pitching arm is decelerated so well by his latissimus dorsi that his elbow actually winds up tucked in next to his rib cage as he finishes his follow-through.
Read the whole thing here: Texas Rangers Prospects: Martin Perez and Juan Grullon