Venturi effect
Small air-only torches normally use the Venturi effect to create a pressure differential which causes air to enter the gas stream through precisely sized inlet holes or intakes, similar to how a car's carburetor works. The fuel and air mix sufficiently, but often imperfectly, in the burner's tube before the flame front is reached. The flame also receives some further oxygen from the air surrounding it. Oxygen-fed torches use the high pressure of the stored oxygen to push the oxygen into a common tube with the fuel.