This video describes what happens to me, and many players, or we RUSH the downswing, or alternately, we rush the backswing. Why would we rush? I think one reason could be that the more we turn our backs to the target, the more uncomfortable we become, so we cut off the backswing. At least I think this is why I sometimes rush. Our brains might not be comfortable with our backs facing the target at the top, especially with longer clubs because we all "know" those shots can get "out of control" more easily. A shorter, rushed backswing creates a feeling of "more control", and gets the swing "over with" to "avoid" mistakes. However, the exact opposite is true.
The check for me is to feel extension at the top of the backswing, not a lot, not overdoing it, but enough to feel a slight stretch and to get the trail hip rotating up during the backswing. Otherwise, with shorter swings the trail hip will more move laterally on the backswing. Thus the other check is if I see / feel my trail hip going up in the backswing. If not, then my backswing is too short, and most likely due to rushing.
The other thing I try to do to stop rushing is to pause at the top with my back to the target, or a 80-90* shoulder rotation. The count is "back, one, hit". I bet folks who rush their swings will find the "one" count at the top to be extremely difficult to accomplish. However, IMO one must get very comfortable with their back facing the target, with a pause, at the top. The pause also allows the body to better organize the sequencing on the downswing. Rushing forces an early post-up of the lead leg for a reverse pivot and also promotes early extension.
Has anyone ever considered that their stance may be causing the rushing of one's swing? If the right foot is straight at address, then upon turning back, the body will feel considerable stress early in the backswing. Thus, the body will want to relieve that stress as fast as possible to start the downswing.
Check out Rob Cheney's stance video below. Then check your feet position at address in terms of the amount of flare and also the width of your stance. Tom and Rob's stance is much wider even on 7 irons than most folks I see hitting drivers. Plus, they both employ considerable foot flare at address.
The stance is probably one of the most overlooked positions in golf. Some people who grew up in the 70's have engrained Hogan's right foot position at address. The straight right foot was supposed to support turning-into the right side on the backswing while keeping the trail knee bent. We now know this to be untrue in the modern swing, especially the part about keeping the trail knee bent because even Hogan's right knee was nearly straight at the top, and most tour pro's right knees are closer to straight than bent at the top of the backswing.
If anyone is still employing the Hogan right foot, stop...stop right now!!! LOL.