Sketch 24.
Well, while A is thus retreating—say, from his old position at A_1 on the foregoing diagram to such a position as A_2, with Black swarming up to crush him—the other corners of the square, B, C, and D, receive the order to "swing"—that is, to go forward inclining to the left or the right according to the command given.
Mark clearly that, until the order is given, the general commanding Black cannot possibly tell whether the "swing" will be directed to the left or to the right. Either B will close up against A, C spread out farther to the left, and D come in between A and C (which is a "swing" to the left) as in Sketch 25, or C will close up against A, B will spread well out to the right, and D come up between A and B, as in Sketch 26 (which is a "swing" to the right).
Sketch 25.
Until the "swing" actually begins, Black, the enemy, cannot possibly tell whether it is his left-hand units (1 to 8) or his right-hand units (9 to 16) which will be affected. One of the two ends of his line will have to meet White's concentrated effort; the other will be left out in the cold. Black cannot make dispositions on the one hypothesis or on the other. Whichever he chose, White would, of course, swing the other way and disconcert him.
Black, therefore, has to keep his line even until he knows which way White is going to swing.
Sketch 26.