The second line takes an injury and survives it by moving fast. Legge's translation shows the subject of the darkening wounded in the left thigh, saving himself by the strength of a horse, and calls the outcome fortunate. The wound is specific and it is specifically not fatal. A thigh is what a person walks on, so the damage is to movement rather than to life, and the remedy the line names is a better means of movement applied at once.
This is the yielding stroke that sits in the middle of the lower trigram Li and gives that trigram its whole character. Held between two strong lines, central and correctly placed in an even seat, it can be hurt without being lost. In a hexagram where the light is under the earth, the second line is the point at which brightness is still properly itself, and the injury it takes is the tax on being visible at all in such a time.
The horse is the classic being practical. Speed is not a virtue here and it is not a strategy; it is a resource that happens to be available, and it is used before the wound has time to settle. Good fortune arrives without qualification, which is rare in this figure, and the reason is that the line does not argue with what has happened to it. It registers the damage, applies what it has to hand, and gets clear of the place where more damage is waiting.