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This phrase refers to the trailing edge of a wing and downwash. If the trailing edge was rounded, the high pressure from below the wing would follow the rounded surface and 'spill' upward into the lower pressure area due to Bernoulli's Principle. This 'spilling' could create turbulance, which is why trailing edges are sharp. The sharp edge allows the high and low pressure air to recombine smoothly, creating smooth flight.