From the description on the card "Chinese Criminals in the Viceroy's Yamen, now used as a jail, Tien-Tsin, China"The prisoners who appear to be Boxers (anti imperialist rebels) are wearing Cangue (a type of yolk) this was primarily a means of humiliation or corporal punishment though it was also used in torture and execution.
Photo by B L Singley circa 1900