How Do You Get A Stronger Arm Swing For A Spike?

First thing, the power does not come from the arms, it actually depends on the core which actually includes Abs and legs. Work on them first. But to answer your question, you should have dumbbells and work for the triceps and biceps. Some push ups and pull ups will help you a lot as well. Pulley system is of great help as well. Arm rotation and windmill will definitely make your arm stronger. Stretch your body as much as you can.