From The Methods of Nonviolent Action by Gene Sharp, quoting from Edmund Stevens (1967):
...each morning an entire platoon of Chinese soldiers would march out on the ice and lowering their trousers train their buttocks towards the Soviet side, the ultimate in Chinese insults. This exercise continued until one morning just as the Chinese assumed their positions the Russians set up large portraits of Mao facing in their direction. The Chinese hastily covered themselves and retired in confusion. There were no repetitions.Sharp includes this under discussion of nonviolent method #30: rude gestures.