RG has seemingly generated a unique URL w/<title> for each line of each of each song listing. Google slurps these up and gives them prominent SERP rankings, though the links are all just doorways redirecting to the main song page. (Albeit, redirecting to an anchor at the lyric queried, with highlighting effect and annotation popup to boot. Pretty friendly UX.)
The tactic described in the article is super shady, but this seems totally legit to me. The entire point of RapGenius is annotating individual lines of hip hop songs, so each annotation is a unique bit of content. I have, on many occasions, searched for a particular line in a song and been very pleased that the first result is a direct link to the RapGenius annotation for that line.
Such as linking back to the main song page with an anchor replacing the second part of the URL, similar to Gist links with #L31-style line number links?
I am not sure I understand why this is considered bad, such pages should have very low pagerank anyway (no incoming links) but they do make sense for someone searching some lyrics' lines.
They are pages with different url's pointing to the same page, without doing an actual redirect. Since url's are a big component of the keywords Google considers, doing that has been an SEO no-no for years.