Early release...causes?

I have seen many players make a good body powered practice swing.  Then they step up to the ball and swing with their arms and hands, leaving their body movements behind.  Then early release, flipping, or other disasters occur.

You need a swing thought that will work every time to make the lead side of your body pull your arms, hands, and club down and through impact.

You can find free instruction videos about this.  I watched one today by Paul Wilson, but I did not like his lower body keys.

Take your grip. Now take the right thumb and tip of the forefinger off the grip, let them hang there. Just keep the pressure on the base knuckle of the right index finger. If you swing like this, it may keep your acceleration a bit smoother and you'll have to keep your hands moving to keep up instead of throwing the club with your hands staying behind the ball.

I've even been hitting balls in practice like that with some success, using it as a practice swing on the course, and swinging in the air like that to help feel the shaft moving. When you have those 2 fingers off the club, you can just balance the grip on the index finger pad, and just use your right hand to keep the club balanced and for a release at the ball. The instinct otherwise is to use the dominant hand to control the club too much.

Alternatively, shorten your backswing to get the club pointing up at about 45˚ towards the target, and don't let your left wrist bend. Overswinging and going past parallel, especially with irons, especially especially with shorter irons, is generally bad for the swing. I'm as flexible as anyone and always tried to use a long backswing and a lot of coil to create power. It works better for me now that I go to 45˚ or less with most shots despite my thin 160lb build, more consistent path and I was making 50% of my mistakes with my hands and shoulders in the highest part of my swing. Plus it takes a lot more effort. I get more power, better control, and worry about fewer moving parts now.

Final thought, weight shift towards the target. That's the other possibility I see, having suffered from casting early on. If you look at a good player at impact (with irons, driver swings are a different beast), their hands are forward but so is their weight.

I'm no instructor but I beat this particular swing flaw so I hope it helps. Hogan recommends the 8 finger grip to get the feel on practice swings so I'm not alone with that.

I just find it interesting that with no manipulations, I can create good angles with the practice swing.  I wonder if hitting on one of those simulators where you do not have to hit a ball (it just tracks the club) would help.  Maybe it would reinforce that I can hit the ball well with that type of swing.

I'd suggest you take a lesson and have someone video your swing while hitting the ball.  I had a similar issue, and learned here from Erik that "feel isn't real", so while I thought my practice swing and real swing were the same, they weren't when compared on video.  People tend to swing harder when they try to strike the ball and that can mess with your timing as well as cause you to tense up muscles that you don't during a practice swing.

I am certain my practice swing is not the same as my real swing.  I am wondering what can I do to make it so.  I also noticed on my driver practice swing, it is easy to produce club head speed and have a full finish.  When I hit a ball with my driver, I stop the club shortly after rehinging on the follow through, and do not complete my finish.