In "Tangled Legs," why does China Iron compare the ditch to a branding iron?

Colonel Hernández explains that the most important function of the ditch is that digging it is extremely hard work. He uses it as a way to break in gauchos to his mastery. This causes China Iron to see that he treats the gauchos like livestock, and when he assigns them to the ditch it is his way of setting his mark on them.