The Impact Ball
 

Is this training aid helpful? Suitable for both hitter and swinger or more for swinger?

This is a rubber ball that you can put between your forearm, you will hold the ball in place for the swing. There are two colors on each side, you need to rotate and see the other side color for backswing and downswing, seems more useful for swinger. Information can be found in theimpactball.com

The ad claims that it will guarantee the hands leading the clubhead through Impact - I don't think so.