Hop Utilization vs. Gravity of the Boil

I've read that hop utilization is decreased with the increase of the gravity of the boil. As I understand you can add half the extract for the boil and the other half at "knockout". Is this a preferred method?

If so, how is it done? You just add the other half of the extract after the boil (when the heat is turned off), stir till dissolved, and let sit? And what if you are using two different types of extract? Do you add half of each for the boil and the other halves after?

It's a lot more complicated than that. Wort gravity itself has no direct impact on hop utilization, but precipitated solids (like hot break) can pull iso-alpha acids out of suspension, effectively reducing utilization.

Still, there's other good reasons for doing a late-extract addition, especially with lighter extract brews as it will avoid carmelization of the wort. Simply add the balance of your extract in the last 10-15 minutes of the boil (you want some boil to sterilize it).