During a golf lesson 15 years ago, an instructor placed a ball with wedges chopped out between an aspiring player’s forearms and told him to swing the club. After a few swings, he could already tell a difference. From this, they created the Impact Ball. 
The Impact Ball prevents several common faults such as flying elbows, chicken wings, scooping the ball, and movement in the lower body when putting. These problems affect 90% of golfers with a handicap of 10 or higher. Correcting these faults, drastically changes one’s golf game.
The Impact Ball focuses on training the body and mind for a solid strike at impact. Most amateurs have their weight back and flip their wrists at impact, but the Impact Ball trains you to shift your weight forward and have a strong wrist position at impact. To choose the correct size, you want the ball that forms a perfect triangle comfortably between your forearms. 
Generally, large is for people over 67” or men, medium is for 60” to 70” or women, and the small is for under 64” or juniors. The sizes may vary depending on height, weight, and build.