Flagellation

Why do some Shia engage in Zanjeer Zani? Is it considered a part of Shi'ism? Sayyed Muhammad Husaini Ragheb	Could you provide me with proper references regarding permissibility of Matam? Additionally, which form of Matam is considered correct, simple Matam with hands or drawing blood through Zanjeerzani? Sayyed Mohammad Al-Musawi
Why is Zanjeer considered forbidden but regular Matam is allowed when slapping yourself hard on your chest can also cause hurt? At what point or in what shape, if any, could Matam by hand be considered disallowed? Sayyed Mohammad Al-Musawi	Sunnis often criticize the Shia for their methods of mourning, whether it be striking chests, cutting or self flagellation. How can we substantiate these actions? Sayyed Mohammad Al-Musawi