“Casting” is a swing fault that occurs when the player throws the clubhead down toward the ball from the top of the backswing. This generally results in one of two effects: First, the player may hit the ground well behind the ball, resulting in a “fat” shot. Or second, subconsciously realizing that they’re going to strike the ground, the golfer pulls up, resulting in a “thin” shot. Either way, the golfer is losing power and distance.
The PGA of America site has an article with tips on curing the “casting” swing fault.