I will agree that the start of the downswing begins from the ground up, but the fundamental action in the golf swing is the swinging of the arms. It is interesting to me how many teachers emphasize the feet, hips, shoulders, etc. with very little mention given to the arms.
In my 25 years of teaching I can say without a doubt the free, fast, flailing action of the arms is an absolute fundamental of playing up to your potential. And yet few students are aware of this. Your arms must swing freely so you are able to apply the clubhead to the ball at maximum speed. When you don't, you are forced to apply yourself to the ball - the swinging of the arms is replaced with a heaving of the shoulders.
If you are suffering from a lack of power or poor balance you should focus on one thought at the top of your backswing: to start down with your arms. Make your arms swing the clubhead down and through the ball.
I mentioned previously that a good player's downswing is started with the legs and hips. But if a player lacks a free arm swing it is a recipe for disaster to think of starting the downswing with the legs or hips. It will lead to an early unwinding - a heaving of the shoulders resulting in a weak glancing blow.
The best drill I have found to promote the feel of a good arm swing is to hit balls with your feet together. From this position it is impossible to make a shot other than with a free swinging of the arms and clubhead. You'll quickly find that any excessive use of the upper body leads to a complete loss of balance. Tee the ball up slightly and use a 6 iron. You'll be surprised how solid and far you'll hit the ball.
There are many "methods" in golf instruction, but the fact is that whatever method he or she used, every good golfer in history has swept the club through the ball fast and freely with their arms.