Seth didn't spend too much time with Alfie, before marrying him off like a medieval princess to secure an alliance, so his lessons on controlling his gas organ were not extensive. But the main thing that Alfie had bestowed onto him, was the importance of pressure.
If he built up enough pressure, then the force of his gas stream would be more than enough to stop the fire from burning its way back to the source. Like when you used a can of hairspray and a lighter to create a flamethrower.
Seth focused on the new organs in his body, the organs that took him a step away from humanity. A gas sack that was rapidly expanding as it devoured the oxygen flooding into his system and mixed it with its own solution to make an explosive new gas. The valves in his throat, that he had to concentrate to keep close, as the pressure built up inside of him.
