How does twain's use of understatement serve his purpose in life on the mississippi?

Two examples will suffice. In talking of a black friend of one of his white characters he says he missed out when tushe "distribution of awards" came around thus alluding to and understating the plague of slavery in the US and then in referring to a licking by his mom he said the lesson was "deeply impressed" upon him ie his backside was smacked but rather than say that he chose the polite but unders tated way to say it.