I'm not sure this is still applicable--if it is, I have missed it somehow. I find Jonathan's blog list to be pretty comprehensive and up-to-date on the topic:
Very often, tricks are discovered in Tableau, and in future releases, those tricks are either unnecessary or they don't work anymore. I'm not sure if this is one of those cases, but I'm interested to learn from someone who does know.
You might be thinking of PREVIOUS_VALUE().
For example, the WINDOW_MAX(SUM(Sales)) will be evaluated for each row in the partition. By using PREVIOUS_VALUE(WINDOW_MAX(SUM(Sales))), the WINDOW_MAX() calc will be computed just once.
Tableau did fix an issue in v8 with WINDOW_XXX calcs, where the WINDOW was being computed over every row, even if you only wanted the first row, as in IF FIRST()==0 THEN WINDOW_xxx(). The workaround was an along the lines of WINDOW_MAX(SUM(Sales), 0, IIF(FIRST==0, LAST(), 0))).
This is no longer required, but I think the PREVIOUS_VALUE() trick is still a valid optimization, when the value is needed for every row.
I understand now, this makes sense. Very nice, thanks Jim!