hi,a problem i have sometimes (casting ,coming over the top)when you get to the top of your back swing ,shift your weight forward then bring your right elbow down close to your body through the swing[right handed golfer} hope this helps.But remember those in the know see the pro or amateur tips ruin your golf,well some.
To my mind, casting is the word used to describe the movement/unwitting technique where you un-cock your wrists rather too soon and throw the club-head at the ball similar to a fly fishing cast.
For most good players, at the transition from back swing to down swing it seems that you can a) see the movement back towards to target with the body and b) notice that the arms drop back down but the wrists have not un-cocked and the club-head is effectively left behind the hands. The longer the club-head stays "behind" the rest of the actions in the downswing, the faster it will have to move through impact.
The truth is that very few people really "cast", but lose the lag in the club delivery way earlier than they need to. I can see this in my swing, at first, things look good, then suddenly all the power is lost by the time I get near the ball.
I think that most players feel that as they start the downswing, the hands pull down from the top and the wrists remain strong to not waste the lag. It's a hard feeling to learn (don't I know it) because practicing the backswing can be as slow as you want to get the club to unusual positions or plane or whatever, but the downswing never feels so easy to slow down and make checks. There is a drill (which was described as "pumping") which can be used for setting the plane or keeping the club-head behind all the rest
1 Start in your normal address position using a short or mid iron.
2 Make a normal backswing and stop at the top.
3 Let your right elbow move down towards your right side and then bring it back up again. Should feel like a pumping action. Whilst you do this, be strong with your wrists, trying to keep the club-head as far a possible behind your arms. Do not un-cock
4 Do this two or three times and on the last one finish your swing.
It's hard work, but can soon produce improvements.
Practice hitting the ball on the range with another golf ball about 9-12 inches directly behind the one you're hitting. Best with irons, not so good with driver where you want to hit up on the ball.
If you can hit nice sweet irons with this technique then you CAN'T be casting. Your hands will be coming into the ball correctly...... before the clubhead.