2B and C responsibility on ground balls

On ground balls in the infield, the 2B and the C are charged with the responsibility of backing up 1B. Who should do it and when?

When nobody is on base, it's good practice for both players to charge over in the direction of 1B to back up an overthrow. However, when runners get on base it gets a little more complicated.

Lead runner at 1B, the catcher can back up on any ground ball. 2B must have 2B bag responsibility on balls hit to the left side of the infield. On balls hit to the right side, the 2B should be moving in that direction in case he is to field the ball or potentially even cover the 1B bag.

Lead runner at 2B, the catcher may not leave as their is a runner in scoring position. The 2B actually should read the direction of the batted ball. If the ball is hit to the left side of the infield he goes to anchor down 2B. Even if there is not a runner at 1B he should go to hold the lead runner from getting a walking lead away from the bag.

Lead runner at 3B. The 2B actually should be sprinting in the direction of 1B as long as a double play is not in order or he doesn't need to anchor down 2B with a trail runner. The throw from the 3B will likely ricochet down the RF corner if the throw is errant. This makes the 2B's hustle important. The catcher? Well I think this one is obvious.

Many times when practicing ground balls we neglect the person who has back up responsibility. The 2B and the C play an important role in this respect.
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