A 1960s photo of Black-Americans training to not react during a sit-in
Back during the time of the Civil Rights Movement, right around the time of the 50s and 60s, people of color would walk in groups into white only places like diners, soda bars, stores etc and ask for service. When they were denied, they would sit and stay in silent protest while around them white people would become more and more hostile.
They were screamed at, crowded, struck, had food, drinks, ashtrays dumped on them, and worse. And these brave people would sit motionless in protest, not reacting as a form of protest to highlight that they were literally not doing anything wrong.