It is very difficult to get our brains to not want to immediately start swinging the arms as fast as we can on the way down in the backswing. The true key to getting effortless power is doing a number of things, but a quick check list would be as follows: make sure that you have your grip in your fingertips (really feel whippy in the hands, but keep control of the club), next with tempo, I like to feel that my swing is almost the same speed until right before impact and then fire through as hard as I can (while keeping your balance). It will take some practice to get it down and is very hard to fight, but will make a huge difference. Keeping the swing smooth and steady will make it easier to keep your sequence and get yourself back to impact. So just keep thinking smooth, not slow.