User blog comment:FairyRaven/Fairy Tail Chapter 423: Do It All for Love, Review and Commentary/@comment-87.202.75.211-20150309114538/@comment-9595671-20150309134546

I recall during the double battle in the GMG that Makarov made a statement that both unlocked it.

From what I can gather intense training is how its normally unlocked, Ultear's magic was just a shortcut. It kinda accelerated the rate of their body's growth to replace the 3 month's they lost in the Celestial Spirit World