They were waiting only for the lightning to become blinding, the thunder to become deafening.
And when the electrical storm was at its height, you will know what happened when those white-clad figures went among the thousands of range-bred beasts, guarded by a pitiful handful of men.

For range cattle are accustomed to a man only when he is mounted; then he is a part of his horse.

It is dangerous for him to go among them on foot; then he is a strange animal.

Many a cowboy has dismounted, rescued a steer from the mire--and had to run for his life.

Thus were those white-clad figures doubly monstrous and terrifying to the herd.
You may have thought that the cowboy wears his revolver for protection against his human enemies, but it is rather for a protection of the cattle against themselves in that strange panic known as a "stampede." Whitey and Injun, riding near the edge of the herd, and bowing against the fury of the storm, did not need Buck Milton's hoarse shouts of warning to make them swing aside.