Impact bags are a great swing aid as they help you to learn about your impact position by stunting your golf swing when it is actually moving at its fastest, providing you with valuable information about the position that you achieved.
The ideal position to be in at impact, for right handed golfers, is to have your weight on your left side, with your pelvis rotated towards the target, your right heel off the floor and this foot rotating towards the target and your left arm should be straight and extended down the front of your left side so that your hands are in front of your left thigh. The club head should be striking the ball when your body has achieved this position so there should be a forward lean in the golf club position, with the handle of the club being closer to the target than the club head. It is this forward lean shaft position that using the impact bag can help you to achieve.
There are two ways that you can use the impact bag. First of all, place the impact bag on the target line, opposite your left foot. Swing to the top of your backswing and work on swinging back down to the bag by rotating your pelvis towards the target and driving your hands over the bag just before the club head strikes the bag. You can also turn the impact bag on its side and place it opposite the middle of your feet on the target line. Swing away from the bag and again on your downswing rotate your pelvis towards the target but drive the shaft of the golf club into the impact bag so that part way up the shaft connects with the upper part of the impact bag. Again this will encourage you to have a forward lean in the club shaft. The club head hitting the bag before the shaft is an incorrect movement that would see you making a flicking action with your wrists towards the golf ball and as a result of this you would strike the top of the golf ball.
Drive the shaft into the upper part of the impact bag and this will give you the feeling of the correct impact position, that you can then begin to replicate when you next hit golf balls.