Have you ever heard a person say that the pitcher is all arm?

Notice in the picture how the knee is lifted past 90 degrees along with perfect balance.  A perfect set up for explosion off the rubber.  Most kids starting out at 9 or 10 yrs old will only lift their knee to about 45 degrees at best with poor balance, and normally goes uncorrected.  By the time they are 12 & 13 it is much harder for a coach or parent to correct.  A poor leg lift = poor power.  Kids that don't get this corrected you won't  see pitching at the high school level.

Try this yourself.  Lift your leg up a few inches off the ground where your knee is at about 45 degrees, and push off with your base leg.  You can't really explode forward, can you?  Next, lift your leg until your knee passes 90 degrees like in the picture.  Now push off.  Can you fell the additional power you now have to explode off the rubber? 

Teach this to the kids at 9 and 10 years of age.  Future coaches will appreciate it.