User blog comment:Miskos3/Fairy Tail 535: The Greatest Power, Review/@comment-5303512-20170514221549/@comment-7418318-20170515204710

It's probably the same as second origin only on steroids. So they can cast the same spells stronger and also they can now use spells they lacked sufficient magic power to cast before.

Of course that's just my opinion, but considering that both SO and FH give additional magic power it would make sense for them to basically work on the same principle, only that FH is far and beyond better than SO.