From "Building A Championship Offense," by Bob Ligouri:"A player/team must have quickness to effectively penetrate a defense. Naturally, blur quickness was an advantage for point guards, but tape analysis showed many excellent teams with average speed point players. They good teams structured opportunities for the perimeter players to penetrate. Ball screens, high post step outs, and flash cuts all provide ball penetration within the defensive perimeter. The ability to break down a defense is critical to offensive success. The top teams structured their attacks to feature the strengths and hide the weak areas of their players. Winners found effective methods to penetrate, even without good quickness at the point guard spot."
I would add to Bob's points, all good ones, that we believe the two most important qualities for penetration are spacing and ball reversal. Man, zone or combination defense -- your offense needs to have elements of spacing and ball reversal available. By spreading the defense out via spacing it makes it more difficult for the defense to close the gaps in help situations. Ball reversal forces closeout situations which can be great for dribble penetration or penetrating passes to the post or players flashing.
