The golf downswing should be powered from the ground up. This is a near universally accepted fundamental in golf. And I don't believe it's wrong; however, it's possible that--like many things in golf instruction--it is misleading.
I am now a believer in pulling instead of pushing in the golf swing, and translated to TGM principles, I suppose that would indicate that I'm more an advocate of swinging versus hitting, respectively. What does this mean? Recall that pulling the right shoulder back in the takeaway and backswing is the correct pivot move versus pushing the left shoulder back behind the ball. Executing a pull takeaway and backswing engages a different set of muscles--muscles in the right side of the back that pull--a more efficient biomechanical motion. Pushing the club back uses a different group of muscles in the left chest and triceps; and this less efficient movement can introduce many errors into the pivot and torso turn. Likewise, pushing on the downswing can introduce errors and rob one of power.
For the backswing, concentrate on pulling your right latissimus dorsi or “lat” (the large wing-like muscles on the sides of your back) immediately back behind you and as far away from the ball as you can. In the downswing, do the opposite with the left lat; it moves first before anything else. I like to think of a “lat dowel” (a wooden dowel that extends through both lats) as powering the golf swing—including chips, pitches, putts, and bunker shots. Thinking of only the lats rotating and moving the arms and club gets you turning instead of swaying, and it really builds up tension across your back that you want to release. It also takes concentration off of your arms and wrists, which is a good thing in golf when you want to swing rather than hit at the ball. The motion gets your right side deeper on the backswing, making room for you to come from the inside, and prevents you from faking a “shoulder turn” using the very independent movement of the shoulders themselves.
Don’t worry about your hips turning as a result of this rotary lat motion—they should! Ignore those who tell you to keep the hips still, as I believe this advice gets more amateur golfers to sway than anything.
This “lat dowel” motion is not unlike trying to start a lawnmower; you would have to pull hard using your lats to get the engine to turnover. Rowing a boat with dual oars also illustrates the pulling-pushing difference: You would pull against the resistance of the water to propel the boat forward using both lats predominantly, and you would push using predominantly both chest muscles to get the oars back into position for the next pull. Trying to propel the boat through the water using a pushing motion of the chest and triceps would be less efficient, because those muscles aren't as big or strong as your lats and shoulders that are designed to pull. The physics of pulling trump pushing nearly every time. Have you ever seen a motorboat pushing a water skier? Have you ever seen a truck pushing its trailer?
So what about starting your downswing from the ground-up? Well, I've found that pulling with my left lat (or really releasing it) automatically engages me from the ground-up. I don't have to think about bumping my left hip and then turning my left hip back; all that happens without thought when you're telling your body to pull down fast with the left lat. You can't pull if you're not connected to the Earth, and you can't pull from the left side if you're not over your left leg.
As a right-hander, I personally like the golf feeling that my right-side Lat muscle is pulling on the backswing, stretching the Lat on the other side, which then pulls the downswing. (And, of course my external and internal obliques are aiding the rotation—it’s unconscious, and it’s called the anterior oblique sling.)
Bottom line: the conscious turning is accomplished from the sides and back, not from the front. And, it happens lower than shoulders, and higher than the hips (which are both just joints and bones, and are just along for the ride).
Your legs should play a supporting role, but the real key to starting down correctly is to engage the left side of your torso (for righties), particularly your latissimus dorsi, or "lat" muscle.
The correct concept involves “pulling” your arms down in front of you using the large muscles of your back and shoulders. You should feel as if there is a point just under your left shoulder blade (the opposite for left-handers) that is the center of this pulling action.
As you start down, make a lateral move toward the target with your left lat. Your left hip and entire lower body will support this move naturally. Now you're ready to unwind.
The latissimus dorsi has the largest surface area of any muscle, and it is capable of producing tremendous power. By stretching it in the backswing and then quickly shortening it in the downswing, the latissimus dorsi transfers and amplifies power up the kinetic chain from the hips to the upper body.
The target-side latissimus dorsi helps pull the golfer onto his target side while countering the force generated by the pectoralis muscles on both sides of the golfer’s body.
To get the feel of contracting the latissimus muscle, simply go to the top of your swing and pose, then drop your lead arm, feeling as if a point in the middle of your left scapula is actually contracting and pulling the left arm down.
Here's the big thought: You're not pushing the club down; you're pulling it down. And you're not using your arms, hands or any other part in the front of your body; you're using your back.
The age-old question is, "What starts the downswing?" Now you know the answer: In the upper body, the back moves the front.
This feeling of having your lat muscles engaged is what you want to hold onto throughout the entire golf swing. That's what keeps the shoulders down and keeps you "in the box." It keeps you connected to your core muscles and all the muscles in the torso, allowing you to use your big muscles in the golf swing.