This is the constant problem when talking to the Neijia people--their use of "internal" is different from most people's. I would just say that what they're talking about is just a more refined and subtle way of training the muscles that we all do. i know that is Kishaba sensei's practice, they don't talk about internal of external, but refer to it as 'koshi' practice, (koshi being the japanese term for the part of the body from the upper thighs to the rib cage, more or less) this avoids any misconceptions.
Anyway. back to your question. Now, this is why it's too bad Mike Sigman isn't in this conversation, and I'm just giving an outsiders exegisis of the Neijia position, but the 'internal' practice they espouse is a very specific training regime that attempts to train the body to work as one piece, utilizing the smaller 'skeletal' muscles (their term, if I recall rightly) rather than the 'external' muscles. For example a punch has very little to do with the triceps, pecs, deltoids, etc, but more to do with the smaller muscles that we usually regard as secondary).
I regret that they use the term 'internal' because it is too ambiguoous and causes all sorts of miscommunications that , in my experience, seldom get resolved.