what is the difference between 'Li De Shi Yong' and Fajin? Is not Fajin a different set of characters?
"Fajin" is 发劲. I think some martial arts talk about "fali", which has almost the same meaning; however in Tai chi parlance it is important to differentiate the two. I have not come across the phrase 'Li De Shi Yong'.
5. Ping heng means balance. Central equilibrium isnt a term that my instructor uses but is probably used to describe something other than balance.
I have heard my teachers use both terms, but I think with different meaning. Without balance, you cannot be calm and rooted. Without central equilibrium, you have no center from which to divide right from left, up from down, front from back, full from empty, quick from slow, etc.