When about a quarter of the Chu army had crossed the river, Han Xin signalled for his men to open the dam, drowning many Chu soldiers and isolating Long Ju with only a fraction of his forces.
Han Xin signalled for his men. This was to open the dam. Han Xin was drowning many Chu soldiers. Han Xin was isolating Long Ju with only a fraction of his forces. About a quarter of the Chu army had crossed the river.