The effect may be stacked up to three times (for a total of +12 conflagratory damage, +6 conflagratory resist, and +3 melee). The effect may be canceled. The duration displayed is that of the 'top' effect. The initial casting duration is 4 attacks. When attacking with a stacked effect, the duration of all is decremented by one (i.e. casting three Hands at in a row and then attacking lasts for four hits - not twelve).
The Hand of Belimawr is a fiery one indeed; it grants you fiery defense and fiery attacks for a limited number of strikes