while in a battle hold right mouse button to bring up shield, then while holding right mouse press left mouse button. it also must be enabled in the camp menu under mod options. I find shield bash annoying if the ai has it all they do is shield bash. I got mobbed by a group of infantry and they beat me with there shields so i couldn't counter attack, i proceeded to die after that couldnt fight my way out. happens alot during seiges and getting caught will in town by the guards.