Not just sword (though thats my personal favorite) but between and among emptyhand and ALL weapons. Each and every one of the biomechanics of emptyhand is reflected in one or more of the weapons and vice-versa. And since weapons are no more than amplifiers of intent, the use of the body--- or inaccurate use--- is likewise amplified. In this way, if I have not learned to move with balance and timing without something in my hand, what makes me think that putting a sword or staff in my hand will somehow overcome this failing. FWIW.
I agree, You cannot use any weapon correctly without correct balence and knowlege of how to move with balence! I think it does help to see the same motion being done with a knife, sword or Dan Bong and then applying it to empty hand. I have found that this really reveals any balence short comings you might have! I think it also teaches how to move around your opponents strenght with very little effort.
Todd Miller