Why do Shias beat upon themselves on 9/10 muharram

Re: Why do Shias beat upon themselves on 9/10 muharram

Bilkul. Islam does enjoin us to practice sabr and to mourn in silence except when an injustice has been done.

earlier I have quoted the quranic verse where we are told to do so, be dignified in mourning but to speak out against injustice.