The eigth and a half doctor was a incaranation of the doctor that exsisted somewhere between his eighth and ninth incaranations.
While the eighth doctor was regenarating he walked out of him and defeated 10 daleks, as it was the time war at the time this event took place. After that he walked back into the eighth doctor and he finished regenarating.
He maneged geting fire out of his head because he was technicly regenerating at the time. It was through this power that he defeated 10 daleks.