I fO1 does not move to cover X7, X7 will receive the ball in a great deal of space. If O1 does cover X7, X6 has an easy option to play the ball forward to X10. If X7 does not pull wide, O1 would be able to cover him and possibly be in a position to prevent a penetrating pass from X6 to X10. A well organized, well balanced defense naturally does not play in a straight line. There are several lines of defense which have to penetrated in order to achieve the ultimate success, a goal.
Penetration is achieved by running with the ball past opponents - dribbling.
It is also achieved by passing the ball through the lines of defense. It is achieved by players not in possession making well timed runs behind the lines of defense to meet the pass. Finally, it is achieved by shooting the ball into goal. O. He plays the ball to X2, as shown, and then accelerates past O. X2 returns the pass to X1, at position X3. The position adopted by O has been penetrated.