User blog comment:Real Godisme/Chapter 479/@comment-5303512-20160328135925/@comment-1776824-20160328141606

And Natsu made it clear that while he had the tattoo, he could use bits of Igneel's power whenever he wanted. The power could be used at any time, he just released it all at once against Zeref, and then was supposed to have lost it. It's 100% obvious that he used Igneel's power for the demolition fist and the roar. This chapter proved that.