I'm trying to box Model a weapon like the guys in the Gif, but I can't seem to recreate his technique. He Extends some faces to create an extension of the Poly. Basically Extruding it out

I've tried recreating him step by step but I keep running into problems.

Anyone know the technique and how to do it?

Gif of Technique he does:


when I try to re-create it:


I get some weird black face on one of the Poly faces


Help is much appreciated