I'll try again:
As a 30 year veteran of karate, I can attest to you being almost correct here. We usually do a 'drop step' or a 'step and then hit' procedure.
BUT: is the use of the empty step the difference between external and internal? If not, so what?? All styles do things different or there would not be different styles.
We are trying to work out the difference between internal and external
in principle, no tin the manner of one technique or other.
So I suggest that the fact that tai chi does an empty step is not what makes it an internal style compared to the drop step of karate.
That is much too simplistic. Anyway, in some kata we actually do do an empty step so is karate partly internal?
Agreed in part, but, to be blunt, so what? Since empty stepping is not the definition of
"internal who cares?
AND,
What happens to the hips once the empty foot is on the ground? A tai chi player must finish the move according to the unified body principle, right?
Does not the unified body principle tell you (in essence) to turn the hips, torso and shoulders toward the target, drive from the foot and straighten the knee all at the same time together, to provide power toward the target?
What you might not know is that these are the exact instructions for a proper 'step and punch' in karate. So how does the
unified body principle create a difference between internal tai chi and external karate?
Apparently you have no experience with Shorin-ji Karate or, at least, the Shorin-ji Karate from Sensei Richard Kim, late of the Dai Nippon Butoku Kai.
He taught us this principle as a guiding principle of our style of karate. If you don't see it in other karate, oh well - that doesn't mean it is an 'internal' rather than 'external' principle.
But please take a look at Tak's karate in the video link provided. He follows this principle correctly as far as I can see. Perfectly. What is moving when his fist stops? Don't forget the
shaking of fa-jing does not count as "moving" either.
SO,
I'm sorry to "not be convinced" but I have had almost as many years with bad tai chi as good karate. I'm sorry if my attempts to define good proper tai chi precisely in non-karate terms forces you to dig deep. I'm sure it is there - let's not just call me a fool for knowing how karate moves, ok?