And if your power isn't what you want even though you're moving into the ball and using linear momentum for your weight transfer, Step 5 will assist you. Using a metaphor, the perfect swing works as smoothly as a child's swing swinging back and forth between the legs of a swing set. But if Mr. Bully picked up the legs of the swing set and twisted them, the swing would no longer move smoothly, it would fly off to the side. This is what happens when you rotate the body while swinging the racket, the racket can't line up into the ball smoothly. Promising young tennis players jump and rotate their bodies dramatically because they want to hit the ball hard.
Force is a product of mass times acceleration.
Young kids don't weigh very much and aren't very strong, so they throw every bit of mass they've got at the ball to get more zip. As they get older they will naturally hit harder due to weight gain and increased physical strength but their jumping and twisting motions will prove counterproductive in adulthood. Power and strength develop naturally as children grow older. Since rotation is counterproductive to success for an adult tennis player, children shouldn't nourish a set of skills that will hurt them in later years. It doesn't make sense, but it happens all to frequently.