I think Manzella is pretty much dead on. In fact, I like to think of my hands being behind the zipper on my pants in the downswing. And if you watch any of the players on any of the tours, they will have rotated those hips so the zipper is starting to get more towards facing the target at impact. In fact, usually the longer the player hits the ball, the more those hips have rotated so the zipper is getting more and more towards the target at impact. Bubba Watson is a perfect example:
Unfortunately, it's easier said than done. But this "combo drill" that I came up with really helped.
Here's some instructions followed by the video.
The first part of the drill is just a left side connection drill. This drill was originally invented by Sam Byrd and was later used by Ben Hogan. Byrd believes the left side connection drill cured Hogan's hook. The left side connection drill was then made popular by Jimmy Ballard.
This drill is pretty simple. Take a golf glove and stick it under your left arm pit (if you're a righty). Next, grab your club without letting the glove drop out of your arm pit. If you do this, you will now feel the upper part of your left arm pressed up against your left chest, upper left rib cage and up against the side of your left nipple.
Now you want to swing without letting the glove drop until the halfway point of the follow through. In fact, you should try to keep the upper part of the left arm pressed up against the left part of the chest from address to top of the swing to impact to the halfway part of the follow through. Then make your finish.
As always, I suggest doing this part of the drill with chip shots first. Then move on to pitch shots. Then move onto punch shots. THEN move onto the full swing. Remember, easiest and simple swings first (chip shot) then once you get the hang of it, move onto more difficult and complex swings until you reach the most difficult and complex swing (full swing with the driver).
Once you've grooved the left side connection drill, the second part of the "combo drill" is to take your right index finger and right thumb off the grip. While I believe that pivot is the most important part of the swing and pivot is the main cause for flipping, you can still flip if you use too much right hand in the swing. Particularly the right index finger and right thumb are culprits of the flip. It's very hard to bend the left wrist at impact if the right index finger and thumb are not doing the flipping. In fact, Ben Hogan used to call the right index finger and right thumb "swing wreckers."
So when you combine the two (left side connection + right thumb/index finger off the grip) it's *almost* impossible to not utilize your pivot to hit the ball. Where you might run into some problems is you might start losing your left side connection to your chest and start moving those arms instead of pivoting to hit the ball. You may also subconsiously re-grip the club with your right thumb and index finger and flip through. But if you keep the left side connection and have the right index finger and thumb off the grip, you instantly feel the pivot doing it's work.
However, it's important to have the practice down. It goes in this order:
1) Chip shots
2) Pitch Shots (half backswing, half follow through)
3) Punch shots (Full backswing, half follow through)
4) Full Swing - short irons
5) Full Swing - long irons
6) Full Swing - Driver
Do it in that exact sequence. You have to truly grasp how to hit each shot before you move onto the next shot doing the combo drill. I believe that this drill will not only give you the feel of using your pivot in the downswing, but will also FORCE you to use your pivot and keep the zipper in front of the hands. You can also use it on an impact bag if you would like.
Here's the video in regards to the "combo drill."