Pitching: Keeping the Foot Under the Knee

Cliff Lee:  Leg up, foot under the knee.
Good balance and good command.

To be successful as a pitcher, actually anything in baseball, good balance becomes a top priority.  If your balance is poor, power and quickness will usually suffer.  This is certainly true for pitchers who have the not so easy job of trying to throw a pitch accurately.  If their balance is off, the ability to be consistently in the strike zone usually decreases.
If a pitcher needs to improve his balance during his delivery, one thing that may need adjusting is the front leg that is lifted during his delivery.  Lifting the front leg allows the pitcher to get his weight back and also will allow the pitcher to develop some power out of his lower half.  However, not all leg kicks are the same as the photos show.  

Dontrelle Willis:  Foot outside the knee.  Back,
head, and eyes tilted back.

Regardless of how high the leg is lifted, it is important to keep the front foot below (or inside) the knee that has been lifted.  In the first photo, Cliff Lee does this well.  Keeping the foot below the knee, as opposed to outside the knee, generally allows the pitcher to keep his back straight and his eyes level.  Keeping your back straight, your head up, and your eyes level are essential parts to balance.
In the second photo, Dontrelle Willis has his foot outside his knee when his leg kicks upward.  When his foot is extended outward, his body, in order to keep his balance, has to compensate by leaning back.  This tilts his back, head, and eyes out of alignment.  Doing so usually causes a pitcher's control to suffer.
Some pitchers can successfully kick their foot out beyond their knee and still have good balance and command in the strike zone.  But it is tougher to do.  For younger pitchers especially, keeping the foot below the knee can be one of those little adjustments that can go a long way in improving a pitcher's command and overall performance.