Not sure, but we definitely saw him practising this in chapter 336. Funnily enough, this chapter lands so well because this is one of the plot lines of this final arc which actually had quite a bit of set up(unlike 99% of the other characters).
Also, again, when the hell did Dabi get this cool?
What happened to my chud villain that got his ass dumped by every hero he ever fought against?
This isn't my Dabi. This is *quivers*, a competent villain. *Gasps in shock and amazement*