The fox moves his hips and pulls free his member. Once he achieves this he wriggles himself in further and makes sure the dragon's mouth does it's work. He's not doing this for a simple hand job. When his hips are in and his member is finally subjected to the chewing as well, it grows hard from the motion of both tongue and teeth. He thrusts while the dragon's gullet seems to get an ever bigger grip on him.