User blog comment:B214/Volume 21 info & spoilers/@comment-205.197.242.172-20160708201350/@comment-24.113.252.213-20160711010249

It's true that Diehauser needs to know how an ability works in order to render it "worthless."

And it's true that [Penetrate] bypasses all defensive abilities.

However, Diehauser isn't using his ability to defend against Issei's attack, so even if Issei used [Penetrate], it wouldn't matter. Issei's attack will land, but the power behind his attack will be rendered "worthless."