Due to certain physical laws, when Sal punched that spot, the firestarter went flying backwards on his behind. He landed with a strangled, "omph," and then he sucked in as much air as he could to re-fill his suddenly emptied lungs. A healthy bruise was already forming, but Mitch didn't have much time to feel it.
True to his blunt nature, Mitch wasted no time in retaliation. Instead of getting up and throwing more punches, normally, he lifted both hands and sent a wave of fire through the air towards Sal.
The boy was too fed up with Sal to wait and fight fairly. The wait for this Niflheimer was over.