Force, Strength and Power

Working my way through the Australian Kungfu Federation's coaching accreditation, I was reminded recently of that good old sports science equation:
Strength + Speed = Power
From the point of view of Tai Chi Chuan, this presents some points of interest. In the Classics, reference is made to "Li" (strength) and "Jin" (power/force) but not really speed (although we do find "zou" - moving). "Power" in this context is defined rather differently.
To an extent, we inevitably fall into the twin traps of semantics and translation. What is the difference between force and power? The word "Li" may be understood as 'brute force' - basic muscular force - and TCC practitioners are not supposed to use it. The last item in the five step strategy ("mian, lian, nian, sui, bu diu ding) translates as "not brute force".
Two of my Tai Chi brothers are also real brothers - Guy and George Burgess. I remember once hanging out with a half-naked George and remarked that his very muscular torso must be the result of "weight training" (by which I meant "body building"). He swore that no, this was just the result of his Tai Chi training. The results of external and internal methods can lead to similar results, although in the case of the B brothers, there's surely a lot of genetics involved as well. It is important to note that, as in the Tai Chi symbol, there is always a bit of Yang in the Yin, and vice versa. So there are elements of external training in Tai Chi Chuan, which may vary from toughening the knuckles on pads and bags, to various forms of resistance training.
The word "Jin" in fact includes the character "Li" as its radical - as per that equation, strength is part of the mix. But if speed is not the other part, then what is? As Mr. Docherty seems fond of reminding us, it  is all about "100 times refined steel" - the long refined product of continuous work, practice, and familiarity. It is developed by working on our skills in Listening, Redirecting and Emitting ("Ting Jin, "Hua Jin", "Fa Jin"). That requires attention, dedication and the application of will power - in a word "Yi".
So, from a TCC standpoint, perhaps we can say
Li + Yi + Practice = Jin (also translated as "intelligent force").