Who teaches "Keep your shoulder in there"

Most kids use their front shoulder and arms to generate power -- i.e., "pulling" or "sweeping the bat through the zone. Keeping the front shoulder in simply lets the hands generate speed from a "pushing" position, while the power from the hips is tranmitted through body rotation.

The simplest analogy is a "one-two" punch -- the left jab requires the front side to stay closed (for accuracy) while the right cross comes through to finish off the job.