thank you for the references. They gave me much food for consideration. It's a pity that I cannot image what the similar phrase in the art of calligraphy means. The phrase about roundness and squareness in taiji dictionary is from Chen Xin's works, I'll cite it below.
I think you are right about "internal/external split", I liked that your phrase.
Chen Xin wrote:
¡°[If it] only [has] roundness, [but] no squareness, this is certainly hua quan (slick) boxing. [If it] only [has] squareness, [but] no roundness, this is certainly ying quan (stiff) boxing.¡±
From what I read, roundness YUAN relates to continuity, smoothness (run4), liveliness-flexibility (huo2); square(ness) FANG relates to the four primary directions (peng-south, lu-west, ji-east, an-north - one of the correlations you may met) and the four corners and to the other things.
From all of this I made conclusion that speaking about "zhi jin" (literally - straight/direct/frank strength) Yang Chengfu may mean a sort of "one-sided" or "merely overt" manifestation of jin.
[This message has been edited by Yuri Snisarenko (edited 11-21-2005).]