These are actually used with Slayer masks (i.e., mask of the dagannoth) and resets them so they confer certain benefits again. When you first get the mask (from TH, or elsewhere) it doubles every 10th drop or so, does extra damage, etc. for the first so many kills, but not subsequently. The spirits of battle resets that counter so you get the benefits once more. It doesn't allow you to reset your Slayer assignment or anything.