The wheel-action pitching machine consists of 1, 2, or 3 wheels that quickly rotate to accelerate the ball forward to the batter. The machine is fed by a person or by a "feeder" (a device created to drop a ball in the chute every 5-10 seconds). Wheel-action machines are easily able to throw different pitches by adjusting each wheel's speed. For example if a 2-wheel machine has its wheels set at the same speed, a straight fastball will be delivered. But if you adjust the wheels so one wheel is slightly slower, then the ball will be rotated and delivered on a curve.
There are critics of wheel-action machines, who worry that these machines are bad for a hitters timing. But if used properly, wheel-action machines can and will improve your hitting. If the batter can learn to load their hand and prepare themselves to swing, a wheel-action machine can actually be great for timing. It's also very important that a batter does not set out to hit the fastest ball possible. That would defeat the purpose of what the machine can help with. Set the speed to a reasonable speed for your hitter, and work on mechanics and creating a fluid bat stroke.
An arm-action pitching machine uses a steel or metal arm that rotates around to deliver a pitch to the batter. A common praise of the arm
-action machine is that it helps a batter develop better timing because the arm rotation closely resembles that of a live pitchers arm rotation. The most frequent complaint of these machines is that they can only throw fastballs. Most often, you will see arm-action machines in professional batting cages or stadiums - not usually in a backyard. Generally, arm-action machines cost a bit more, though, some of the arm manufacturers have recognized this and have offered a more affordable option.