Which part of it? Why you would want to get them, or why Google does not want you to get them?
Links are the primary determinant of rankings in competitive searches. (Competitive searches are high value ones with lots of savvy folks angling to win, or ones where there are other players for the search term with built in advantages: for example, [credit cards], [buy viagra online], or [facebook] are competitive searches, [patio11], [elementary reading bingo cards] or [how do i sort an array in ruby] are not.) Thus, you want links.
Most methods of actively acquiring links are extraordinarily labor intensive: they boil down to sending emails to folks asking for them to link to you, possibly with a sweetener to the deal (such as money, content, a reciprocal link, etc etc). Google likes this state of affairs, because as long as links are hard to get, then the number of links you have is a good proxy for either quality or prodigious effort expended in ranking. Either works for Google -- it makes spam less cost-effective than buying AdWords ads, and there might be gains to the user experience, too. (A less jaded individual might flip that sentence.)
If you're on a campaign to get links, ideally you'd want that to scale out of proportion to the time invested, for the same reason you want users, sales, etc etc to scale out of proportion to the time invested. Google wants exactly the opposite, for the above reason. Accordingly, virtually any tactic which repeatedly results in scalable link generation will eventually be discounted, regardless of whether it improves the user experience or conforms with Google's guidelines, which they enforce in an arbitrary and capricious fashion.
One example of this is widgets, which do result in scalable link acquisition. See the video for what I think of them.
Thank you Patrick for the explanation. I'm trying to think of the ways in which a widget can create a link on a site where it's installed. I'm thinking:
1) Javascript, buy making a GET back to the widget server, and then inserting the link into the DOM of the widget host page.
2) Iframe.
3) Flash or something like that.
What am I missing? Can't Google easily ignore a part of a page that's generated by Javascript (their bot doesn't process it anyhow, right), and also ignore Iframes or Flash?
You can add my 'empirical' confirmation of your datapoint. We did the same thing with a webcam-in-a-frame using the webcam component, the link was left in-tact in almost all cases.
