I do the same method as Calder on alot of Saisons and IPAs, I usually wait until the krausen is just starting to fall before I add the boiled/cooled sugar water.
I like this method for a few reasons, it gets your yeast to work on the more complex malt sugars before plowing through the simple highly fermentable sugars. Yeast can get lazy and ferment the simple sugars first, by holding these off you ensure that the yeast works on the more difficult fermentables, then the simple sugar is cupcake city.
This helps in my Saisons to ensure that they are bone dry, and avoid some hot alcohols in beers 7%+, some goes for IPAs/DIPAs.