Late Malt Extract Addition — A Primer
Better Beer with Late Malt Extract Additions

I am going to try the late malt extract addition method on my next brew. I’ll probably do about 20% of the DME or syrup to get the hops going, and then finish off the extract for about 15-20 minutes. Another good post about it is right here:
http://menuinprogress.com/2007/08/on-importance-of-late-extract-addition.html

October 8th, 2008 at 2:00 pm
I didn’t even consider adding a bit on the outset, I simply add the entire extract in the last 5 minutes. Otherwise, all that’s boiling the entire time is hops.
I put the extract in just long enough to bring it back up to a boil, and done.
What’s the benefit of adding 20% to “get the hops going”?
October 8th, 2008 at 2:09 pm
From the first link:
The sugars and enzymes in the extract aid in extracting alpha acids (bitterness) from the hops. Boiling hops with a small amount of extract will result in smoother hop flavors and appropriate bitterness that you can’t achieve with plain water alone.
So that’s why I was going to do it that way.