Arm action can vary by player or by the position they play. Regardless of your position, the goal is to be fluid and display a continuous arm action through your target. Your step, shoulders and hips should be in line with your target.
Another key is your hand separation. Separate your hands (baseball from glove) with the thumb of your throwing hand pointing down, moving in a circular motion until your thumb is pointing to the sky.
Lastly, focus on accelerating your arm through your target and not decelerating your arm motion.
To work on your arm action, try the figure 8 drill. This drill helps maintain good fluid movement and continuous arm action while accelerating through the target. Move the ball with your throwing hand in a figure 8 motion a few times. This should be done with your arm extended in front of your body. As you finish the lower side of the figure 8 motion with your throwing arm, bring the baseball up to the sky, step and throw in a normal motion. Make sure to maintain the momentum created from the figure 8 motion and accelerate through the finish.
Remember, proper arm action can help you develop an accurate and strong throw in baseball.