Im not really sure but looking at the article you've posted, im guessing it's a transition between colours so rather than a highlight or shade as such just using different colours.
ok, for the sake of CofC, lol, I will go get a copy of WD this weekend; honestly I have not bought WD for years since I do not think that I can learn much about painting from it. Guys, I will probably scan the page in a bigger size or try to summarize into 1 or 2 points and report to you guys.
Rather than buy it you should just flick to the relevant page you know which one it is now lol. I think the WD have been trying to show simple techniques for painters but not always in much detail so it's not always so interesting for the painting community.
From the photos i honestly just think it's blending between two colours which don't highlight or shade specifically (but can give that effect). I just think GW are trying to put a technical name to a technique already used and done, but speculation can be a dangerous