If you want a full explanation, look up Helmholtz resonance. But the gist of it is as the intake valve closes, the air still moving towards it gets stopped abruptly creating a pressure wave which bounces back down the runner. Then as it comes back the other way if it reaches the valve when the valve is open again for the next cycle, there's a slight pressure increase in the combustion chamber which means more oxygen, allowing for more fuel, meaning more power.