Focus on building a strong base with your legs, loading your body weight, hitting with a high contact point, using wrist snap, and rotating your body after impact to generate more power and aim for a high bounce on the back glass.
This covers some great points if you want to add more power to your overheads. The coach shares his personal approach, focusing on using your legs and body rotation effectively. There is also good advice on wrist snap and aiming for a high bounce. It gives you clear elements to think about when working on your smash technique.