This is the only firm line the whole figure possesses, and in The Army that fact outweighs everything else about where it sits. A yang stroke occupies an even position, an irregular placement by the standard rule, yet it stands centrally within the lower trigram, and here centrality wins the argument outright. Classical readings of Hexagram 7 treat this line as effectively the commander for the entire structure, the yielding ruler above depending on it rather than the reverse.
When this line changes, the middle stroke of Kan becomes yielding, and since every other line is already yin, the lower trigram becomes Kun entirely, matching the upper trigram Kun above it exactly. The resulting hexagram is Number 2, The Receptive, earth doubled, an image of complete devotion and yielding with no independent firm line anywhere in it. That removing the army's single point of firm command should produce the hexagram most associated with pure receptivity is a direct and almost mechanical demonstration of the hexagram's own premise: take away the one capable commander, and the disciplined multitude of The Army collapses into undifferentiated followership, with nothing left to organize it. Read together with line five, which occupies the ruler's own seat without the firmness this line supplies, the pairing states as plainly as the hexagram ever does that authority and position are not the same thing.