User blog comment:IamJakuhoRaikoben/Chapter 464: Natsu vs Zeref, Review./@comment-153.91.253.204-20151130200254/@comment-1776824-20151202224045

What was stated was that he could only use Fire Dragon King Mode once. He never said that he couldn't use bits and pieces of Igneel's power here and there (which he has clearly been doing, evidenced from the spell names being "Fire Dragon King's" and his appearance becoming more draconian), just that the power was finite, and that releasing it all at once in burst - which is the Dragon Mode - is what drains it completely. It's been shown that he can use Igneel's power independent of releasing it all at once.