Casting is a term used to describe how golfers unhinge their wrists well before impact in a misguided effort to scoop the ball into the air. They look like a fisherman casting a line. Unfortunately for those golfers, there's no 10-pound trout on the other end of this cast: They'll likely hit the ball fat or thin.
For solid strikes, your wrists should not unhinge fully until after impact. Get a feel for this with my punch drill. Using a 7-iron, hit punches with an abbreviated finish, keeping your hands leading the club well past impact (right). Don't let the shaft flip past your left arm. Your divots should start in front of the ball.
Hit 10 punch shots this way, and then re-create that feel when you hit a normal full shot. If you don't hit it solidly, you're probably casting. Go back and hit another 10 punches to ingrain the feeling.