They broke the barriers that held back the Yellow River to cause major flooding in the path of Japan's advance.
Chiang Kai-shek ordered the destruction of the walls that were constructed to contain the river. It was done without warning to Chinese people in the regions, so that the Japanese army would be caught by surprise. It also caught the Chinese people by surprise. A half million were killed in the floods and around four million had to flee their homes and villages. Such a costly act in their own territory showed just how determined the Chinese were not to let Japan overcome them.