THIS picture shows the value of the second line of defense when the primary line has been overcome or split apart. The offense, by faking a sweep toward their left, opened an enormous hole in the opposing line. The runner (see arrow) is taking full of the opening but a defensive back blocks Had the flayer who was assigned to "clean up" this hraarance cornea out his assig runner, by swerving to his right, would have enjoyed free territory.
Because of his importance to the defense, a player of rugged build and possessed with natural ability to tackle is detailed to "back up" his linemen. Harvard vs. Princeton 1921.