This is just what I think happens so take it with a grain of salt!
I think mana is just memories taking physical forms, they stay in the air when they form, so when you use magic, the cost it has is that you're burning those memories, even the ones that aren't yours, (mostly the ones that are floating in the air and present around you), the stronger the memories are, the stronger they burn. So going ablaze is just burning throught the memories that surround you so quickly that you also start burning the ones inside you too, I believe that inside your body and inside the creatures in general, mana is present in the blood (and saliva, mostly fluids I assume). In times of need a creature concentrates it on a liquid like blood, and if there happens to be a lot of mana on it (like going ablaze), the moment that liquid touches the air the reaction happens so quickly it cristalizes instead of burning.
We see this as Alon bites the hand of that guy, or when Luke is stabbed on the past sequence, etc.