Originally Posted by
Phil2016
As mentioned here, be ready and available to go online exactly at the 24-hour mark, where P and Exit row seats will become available to you.
This is somewhat misleading. NW does NOT always let non-elites select P and E seats at the 24 hour mark. Anecdotal evidence suggests that the more full a flight is, the more likely the seats will be released right at 24 hours. But I've never seen a thread confirming the exact criteria. There are many threads on this subject.