Casting a golf club means that when you swing, the club comes out and around to the left. It is similar to a right handed batter pulling a ball down the 3rd base line. When you swing your hands and the club form a 90 degree angle. That 90 degree angle is supposed to be maintained through the swing to give your club head a spring motion as it hits the ball. That spring motion is where all your power comes from. When you cast the club you lose all that angle and all your power. To prevent casting use a simple drill. Set a club down beside your ball that you are going to hit pointing straight out, line up as normal, then turn your feet in 45 degrees. When you swing don't allow your body to rotate to the left. Keep your body straight. This will force you to swing with your hands in the proper position. By doing this drill you will help eliminate casting when you swing.