This is an excerpt from Survival Guide for Coaching Youth Softball by Robert Benson & Tammy Benson.
The magnet principle is a basic concept that can be used to help young players learn defensive coverage. In this strategy, the players pretend that the ball is a gigantic magnet and that they are all pieces of metal that are attracted to it. Wherever the ball is hit, the defenders need to start moving to it, not to see who can get to it first, but to provide backup and assume supportive roles once the ball is picked up. If a defender comes to a base before she gets to the ball, then her responsibility is to be on the base. Make sure you explain that only one person is allowed to pick up the ball; otherwise, you may have several players tackling each other to get to the ball. At other times, you may see several defenders standing around the ball in what appears to be a board meeting to decide who will pick up the ball.
To begin with, make sure your defenders know where their basic setup position should be on the field (see figure 7.2) when they're preparing for a hit. They can't move to the correct coverage position if they don't start in the correct place. The pitcher starts on the rubber and the catcher starts behind home plate. The first and third base defenders should be approximately 3 feet (9 m) in front of their base and 3 feet from the foul line. The second base defender and shortstop should be about half way between the bases and just behind the base path. The center fielder should be directly behind second base and about 20 to 40 feet (6 to 12 m) from the base. The left fielder plays at about the same depth as the centerfielder and should be half way between the shortstop and third base defender. The right fielder plays at a similar depth and should be between the second base and first base defenders.
Then, to lessen your frustrations during a game, walk through various situations and discuss how defenders should move to provide coverage for a hit. For example, you may want to go over what the defensive players should do if the ball is hit between the defenders at first and second. The second-base defender moves to the ball with the intention of fielding it. This defender should continue moving to the ball unless the first-base defender yells that she has it (meaning that the first-base defender will field the ball). In this case, the second-base defender continues on to first base and covers the base (see figure 7.3). The second-base defender is now ready to receive the throw at first from the fielder (i.e., the first-base player). If the batted ball goes past the first-base defender while she is attempting to field it, the second-base defender will be in position to back up the first-base defender and to field the ball because she was already moving to the ball and she is slightly behind the first-base defender. The first-base defender can then return to first base and get ready to receive the throw.
During any of these situations, the shortstop runs toward the ball as well. However, the shortstop will hit second base before getting to the ball, so she needs to stop at second and take responsibility for that base. The right fielder starts running in toward the ball and takes the ball if it gets past the two infielders. If one of the infielders gets the ball before it reaches the outfield, then the right fielder will run toward first to provide backup and to cover any overthrows (which are very likely to occur).
This is an excerpt form Survival Guide for Coaching Youth Softball.