Why place the stirrup at the ball of the foot?
The iron (stirrup) should be placed at the ball of the foot, perpendicular to the girth with a slight turn towards the inside slightly in front of the outside of the iron. This allows for more depression (downward movement), more flexibility and more sensitive feel.
If the stirrup slips back to in front of the heel of the boot then this is called "home". The heel is not flexable in this position and cannot make good contact with the horse. The heel cannot depress into the downward position that creates the "heels down" position.
If the stirrup moves to the front of the foot near the toes the heel becomes weak and this position undermines the riders security. This position gives the foot too little of stirrup support. With young riders this foot position creates weight in the toe and the heel lifts instead of going down. (watch for "why heels down")