There has been some discussion on the same topic on FTLOP recently in this thread here
Essentially, Roy is suggesting that as well as the option of filtration, the winemaker also has the option of varying the amount of time the wine spends in contact with the grape skins and grape mass to leach the tannins out. If you want to make a fruity port that still has some structure but not the tannic wallop required for very long term aging, then you crush quickly and ferment immediately without allowing the must to mix with the juice for very long.
On the other hand, if you want to make a tannic brute that will require 40 years to mellow into something enjoyable then you leave the juice and must in contact for a long time under refrigeration so that the fermentation doesn't start until you have completed the extraction to the level that you require.
Incidentally, does anyone know where the fortification process takes place? Is the aguadente poured into the lagar with the fermenting must or is the fermenting must emptied out of the lagar into barrels and fortified in the barrel? Is the brandy poured into the juice or the juice poured into the brandy?
Or is this getting a little too geeky?