What's the difference between "arm" or "wheel" machines?
This is the style by which the pitch is thrown. "Wheel design" machines either uses 1, 2, or 3 spinning wheels to propel the ball. Generally the more wheels, the wider variety of pitches that machine can throw, such as curves, sliders, forkballs etc. "Arm-action" machines from the Master Pitch company use a steel arm which rotates around to hurl the ball. The advantage of these machines is the batter's ability to see the ball for a longer period of time, and work on timing as it's released from the arm.