What do Shi'a Muslims do in Ashura? And why they do it?

Why do Shi'a Muslims hit chests, slapping cheeks, hit shoulders with chains and, hit heads with swords and shed blood during Ashura? These reprehensible things forbidden by the Prophet (peace be upon him) as he did not tell his followers to make something of it or close to it, for the death of a great, or loss of a Shahid, whatever the amount and status. > عَنْ أَبِي هُرَيْرَةَ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه > وسلم:‏"‏ اثْنَتَانِ فِي النَّاسِ هُمَا بِهِمْ كُفْرٌ الطَّعْنُ فِي > النَّسَبِ وَالنِّيَاحَةُ عَلَى الْمَيِّتِ " > > It is narrated on the authority of Abu Huraira that the Messenger of > Allah (may peace and blessings be upon him) observed: Two (things) are > found among men which are tantamount to unbelief: slandering one's > lineage and lamentation on the dead > > \[this hadith in Sahih Muslim ] And there is many of evidence that prevent such acts... *So my questions:* Why they do it? What reasoning and argument do they use to support their practice?