The yoi at the begining and end of the Heian Katas' I think are just a formality, after all they were developed by Funakoshi Sensei to enable them to be taught in mainstream schools.
The Yoi for the advanced katas' however I believe to be 'Kamae'

Kase Sensei used to say that ANY possition could be 'Kamae', a possition that would enable you to either block or attack; take for example the different 'Kamae' that he adopted, left hand by his right hip, right hand held high above his head, he would use the left hand as a sweeping Shuto, that would deflect an attack, then come down with his right hand as a counter; other times his left hand would be held high in front, with his right hand held low behind, when the attack came, he would lower his left hand to block and counter using 'Hente', and then bring his right arm in with a 'Hito'.
The Yoi for the advanced Kata I think is for this reason, and that every Kata served a different purpose, e.g. in this situation you use this Kata, for that confrontation you use that (well...............maybe, but you get my jist

).
I do'nt know, what do you gyu's think.
