It uses the same pool of resources as a surge, but otherwise it is its own action. You have to have the mythic spell lore feat or the mythic spellcasting path ability to learn the mythic version of a spell. I believe you can take it for spells you might not know (but may learn from purchasing or leveling at a later date) but it does have to be a prepared spell/spell known.
You choose at the time of casting if you want to use the mythic or regular version of the spell.
However!! If you have the Archmage or Heirophant ability to cast any spell from your list, you could use it in conjunction with a known mythic spell at the cost of 2 mythic points (1 for each ability, plus any more from augmenting at higher tiers).
Hope that helps!