Let your rapidity be that of the wind,
The simile is doubly appropriate, because the wind is not only swift but, as Mei Yao-ch`en points out, "invisible and leaves no tracks."
your compactness that of the forest.
Meng Shih comes nearer to the mark in his note: "When slowly marching, order and ranks must be preserved"—so as to guard against surprise attacks. But natural forest do not grow in rows, whereas they do generally possess the quality of density or compactness.