Found the answer I was looking for... from a man on the inside.
Although his answer was, in parts, too technical for me to digest, I did get a general picture ax well as some interesting detail...
Basically....
Each IRU remembers last position (even on power down) and uses the latitude component of last position (in the absence of a new PPOS latitude) to compute Earth Rate. If the last position was not too far awry, the earth rate computation should not be too far off... at least under normal circumstances.
Earth Rate at high latitudes is minimal, so has to be looked at for a longer period to get an accurate measurement. Withholding a good PPOS latitude will not help. Therefore, switching directly to NAV and not entering PPOS serves no benefit.
Thanks.
Regards.
Q.