In order to create power on your downswing, you need to ‘get behind the ball’ at the top of your swing. This involves rotating your shoulders and hips away from the target, so the majority of your weight shifts to the back foot at the top of the swing. Once you have wound your body up behind the ball, then you can shift your weight towards the target and fling your arms to hit the ball. Where a lot of people go wrong is they never quite get the lateral motion towards the target on the downswing. They either come over the top with upper body or hang back and use only hands and arms to hit the ball.
Ideally, you need a little lateral motion towards the target to get the weight set in motion towards the target and then let arms swing through to finish.
Here is a drill to help you learn how to sequence your downswing, will help you create more clubhead speed and power.