The SEO Halo Effect
By the definition of Halo Effect:
“The halo effect is a cognitive bias that occurs when a person's positive or negative traits seem to "spill over" from one area of their personality to another in others' perceptions of them.”
So in case of SEO, when you achieve a good rank from a single but competitive keyword, then it makes possible that you could be able to achieve good rankings from other new keywords much sooner than it took at the earlier keyword.
As the world is well aware of the fact that the site takes a longer to achieve a good rank in Google as compared to the other major search engines like Yahoo and MSN, that’s just because the Google search engine tries to block the spammers, for that it has to wait a long to see if the site is spam or not.
So when you achieve a rank in Google Search Engine Result Pages(SERP) by a keyword it makes the remaining task easier, that is, it helps other keywords to achieve a competitive position in SERP.
However when a site gets good ranking from some keyword, then it becomes easy for a website to achieve competitive positions with the aliases of the particular keyword.
So the moral of whole story was to achieve a position by a particular keyword and then let the Halo effect work and watch the magic, in context of Search Engine Optimization.