Two of the biggest components to a good swing are separation and connection. This may sound crazy because they are directly opposite terms. However, both are a vital part to power generation in a swing. I was recently asked by another instructor how to explain to kids the difference, so here is my attempt.
Simply put, separation is trying to get the lower body to rotate before the upper body does. Take a look at Javier Baez below. His hips have started to turn to the pitcher as his upper body stays oriented in the opposite direction. Look at the creases in his jersey.
This separation of the lower and upper body is important for a number of different reasons in hitting. First, as I stated earlier, this is a huge part of power generation. Think of the lower body as the engine of the swing. Rotating the hips and using the biggest strongest muscles in the body to initiate movement helps the upper body move faster.
Separation is also vital in pitching for the same power generating purpose. Look at Aroldis Chapman below. Notice how his lower body has begun opening up towards home plate but his shoulders are still closed. Look at the pinstripes on his jersey and notice how they are angled back towards his left shoulder.
This action of separating the upper and lower body turns the body into a coil loaded with potential energy. When the upper body does fire, it comes around with much more speed than it would without separation. In hitting this means batspeed, in pitching it means velocity.
Connection refers to the ability of a hitter to keep their hands connected with the body during the initial stages of upper body rotation. I use the back shoulder as the the connection point for the hands. Notice how the distance between the hands and the back shoulder does not change throughout the first half of the swing below.
Look at the picture below. Notice how close the hands are to the back shoulder.
Connection of the hands with the body is a vital part of the swing for 2 reasons. First, just like separation of the upper and lower body, it provides for more power. Allowing the hands to travel along with the body allows the biggest strongest muscles to create power. Many kids will lose this connection and will try to generate power with the arms. Essentially, the hands and arms should go along for the ride for the first half of the swing.
The second reason that connection is important is bat path. Allowing the hands to connect with the back shoulder allows the back side to drop and puts the hands in a better position to get the bat into the hitting zone sooner. In addition, it makes it easier for the hitter to create the slight upward bat path to make consistent contact.