When you cast a spell 'for free' it costs (0) mana and requires no additional costs either. 'For free' is synonymous with 'for (0).' You must still meet the threshold requirements to cast the spell. If you cast a spell that costs X 'for free' and you can choose the value of X, e.g. Arcane Barrage, then X must equal zero. But, if the spell itself specifies the value of X, e.g. Twist of Fate, then X = whatever is specified by the spell.