Back Side Commitment
This is often called "squishing the bug", but the term implies that you must keep some weight on the ball of your back foot. This violates the tenants of hitting coaches who teach that you should hit with your weight on the front foot. Regardless of the batter's front/back foot weight distribution at contact, the point is that prior to the swing the rear foot should point toward the plate, and through the course of the swing twist and finish pointing toward the pitcher.
The knee should also be bent to near 90 degress.
This twisting action gets the hips into the swing. The hips are key to increasing bat speed and generating power from the legs. Unfortunately, a player with good reflexes, quick hands and good hand/eye coordination can survive through AAA without good back side commitment. However, the increase in pitching velocity at the majors level makes it difficult to catch up to the pitch without "squishing the bug."
Front Side Closure
While the back side is twisting, the front side should not. It should stay near the "closed" position. Closed means that the front foot is pointed more toward the plate, than to left field. Once the front foot opens, hips and shoulders tend to follow. It's great if the hitter wants to pull every pitch, but as the player progresses up the ladder of competition, the pitchers will catch on and pitch to the hitter's weakness, the outside pitch.
Head on the Ball
I think we can still call this "Mike to Ike" without offending any hitting gurus. Like the front foot, the head does not twist as the back foot, hips and trunk. Mike refers to the front shoulder, or where your chin should be before the swing. Ike refers to the rear shoulder, the position of the chin at the finish of the swing.
Body Stays Centered
With the twisting and weight transfer, the body must stay centered. The body should not fly forward with the bat head and end up over the front foot.
Balance and Extension
The arms should be at (or at least near) full extension at contact; but not before then. Quite often, a hitter will extend their arms early. This is called casting. If the arms are extended too early, and the pitch is on the inside half of the plate, the batter must lean back on their heels to keep from hitting the ball on the bat handle. The result is a loss of balance backwards, which results in the batter catching his balance with a small step back, usually with the front foot
At the completion of a swing, the weight on the front foot should be centered on the ball of the foot, in a very wide stance, leaving the batter in a good balanced position. Also upon extension, and through contact with the ball, the bat head should be flying toward the pitcher, and not immediately whipping around the batter.
In order to stay balanced, extend properly, and swing through the ball, the elbows need to stay bent and close to the body. Keeping the hands "inside" the ball, the bat should be gripped with fingertips, not the palm, and the middle knuckles should be nearly aligned, which in turn keeps the wrists aligned and acting together to snap the bat at the ball.