But after awhile they had to climb over a ridge, and the horses were picking their own way, and the horrid one got off to one side, where Mary V could see him out of the corner of her eye.

And he was not watching her very closely, and the gun was not pointing at her as she naturally supposed it would be, from what he said.
So Mary V very carefully turned in her heel, and watched her chance, and gave Jake a kick in the ribs.

And Jake did exactly as a person expected, and gave a big jump against the horse of the boy.

And the fat one did not shoot after all, because he thought it was Jake that did it himself.
So Mary V, having reached into her riding shirt and got her gun, whirled Jake around and took a shot at the fat one before he saw what she meant to do.

And she hit him in the hand where he was holding the gun across the saddle horn, which was careless of him, but, of course, he never _dreamed_ that Mary V had a gun and would use it.
So the gun dropped on the ground, and the man tried to grab his hand and his side at the same time, because the bullet hit his side too.