Great Preponderance opens with an act of extreme care. Legge's translation describes mats of white mao grass laid under things set on the ground, and gives no error as the result, while the commentary on the image says the line shows a proper sense of what lies beneath. Mao grass was a common material and mats made from it were cheap, so nothing about the gesture is grand. What is notable is that anybody bothered. A vessel set directly on the earth would have been perfectly serviceable, and the subject of this line will not put it there.
The care answers the figure as a whole. Four strong strokes stand in the middle of the hexagram with a yielding stroke at either end, which is why the recurring image is a ridgepole thick through its span and weak where it rests. A structure loaded this way fails at its bearings, and this line is the lower bearing. The Great Symbolism reads the trigrams as a marsh whose water has risen over the trees standing in it, an excess that submerges what ought to be above the waterline.
By the ordinary rule the stroke is incorrect, a yielding line in an odd place, and in a time of excess a weak beginning would usually be the first thing to give way. The mats are what removes the fault. The line is not asked to be strong, which it cannot be, but to be scrupulous about the point where weight meets ground, and that scrupulousness is treated as sufficient.