to make a difficult and courageous decision to cut off a part of something to save the whole, like a hero cutting off his own wrist after a venomous snake bite; to act decisively in a crisis

The group of Chinese soldiers who defended the Sihang Warehouse in Shanghai against Japanese forces in 1937 during the early stages of the Second Sino-Japanese War

Literally meaning a brave warrior cuts off his own wrist to stop snake venom from spreading; used to describe making a quick, decisive sacrifice at a critical moment to save the whole.