a yellow square, indicating this line as the constituting ruler of the hexagram Six at the beginning means:

It must be checked with a brake of bronze.
Perseverance brings good fortune.
If one lets it take its course, one experiences misfortune.
Even a lean pig has it in him to rage around.

If an inferior element has wormed its way in, it must be energetically checked at once. By consistently checking it, bad effects can be avoided. If it is allowed to take its course, misfortune is bound to result; the insignificance of that which creeps in should not be a temptation to underrate it.
A pig that is still young and lean cannot rage around much, but after it has eaten its fill and become strong, its true nature comes out if it has not previously been curbed.