User blog comment:Jopjopjop/Volume 18 Spoilers/@comment-106.76.119.69-20140620090008/@comment-16281343-20140620090656

No detailed explanation has been given yet.

But my understanding on that one is that Penetrate ignores defense.

That's why Issei was able to punch Rizevim even though he's using he's BB because Penetrate ignored/rendered the defense from SGs given by Rizevim's ability.

That's also why Issei was able to see Gabriel's clothing because it ignored Gabriel's clothes which is also a form of defense.

~Ofc, this is just my understanding of that topic with the given infos.