The Serratus helps pull the shoulder blade up against the back when doing overhead movements. Think of your arm as a diving board and the serratus as the support system. The neck/traps will try to support the arm if the serratus is weak or not firing properly, which is really common now because of sitting in chairs all of the time, bad posture, and lack of use. This creates a bad scenario where you are trying to support a diving board (your arm) from the top instead of the bottom which can create tension in the trap, neck, and shoulders. I hope these activation techniques help which you can use to help strengthen your serratus also!