The chapter is about private men of the Han who stood outside the official order and rescued people in trouble at the risk of their own lives, such as Zhu Jia, who hid hundreds of hunted men. Sima Qian admits that their deeds often broke the law, but says
其言必信,
其行必果 'their word is always kept, their deeds always carried through'. Its sketch of one of them as
短小精悍 'short but sharp and tough' is the source of that idiom.