The idea of adding the sugar after boil is this: Some yeast will go all crazy eating the complex sugars and we give them from the wort. After awhile, they get tired and start to slow down (the krausen starts to fall). By giving them simple sugars at this point, it will give them something easy to eat and wake them back up, so that they will attenuate better.
I have only used this technique and had it make a noticable difference with WYeast 3787. I added the sugar in increments. Say the recipe called for 2 lbs. I would add 1 lb once things slowed down (4 days or so). About another 4 or 5 days later, I would add the other.
You only need enough water to cover the sugar when boiling it.