Two-step external linking is the process of requiring a second click, or at the very least a page view, before a user is allowed to leave your website by way of an external link. This differs from redirect based external linking whereby external links are passed through a redirect that is automatically processed without presenting the user with another page view.
Two-step external linking has been in use for well over a decade for some types of sites. For legal and liability reasons many websites, be they government, medical, or children-aimed websites will often warn users when they are leaving the site to view content on an external link.
There is however now another reason to do this.
Webmasters have an onus on them to insure that their websites do not become havens for spam. Search engines such as Google have long insisted that webmasters avoid linking to "bad neighborhoods" as you can be held liable for who you link to. This creates an issue with sites that have user submitted content, such as forums or blogs that allow comments, because your users may be submitting bad links to your site.
Some years ago the search engines, lead by Google, released the rel="nofollow" anchor tag attribute that allowed a webmaster to mark a link as being non-vouched for. This was to be used for links that were not editorially reviewed by the webmaster and could contain questionable content, or for paid links. For their part search engines would not hold a link against a webmaster if it had a rel=nofollow attribute and they would not pass any link weight (aka PageRank) through that link.
So, if you had a site with 20 link weight points (and yes, this is a vast simplification) and you had 10 links on this site that were normal links, such as your menu to other parts of your site, and 10 links on this site that were nofollow links, then the 10 nofollow links would be dropped from the equation and each regular link would just get 2 points of link weight sent through (total link weight / number of links).
Recently Google quietly changed how they calculated this. Whether or not other search engines (all of which have link popularity components to their algorithm) did the same is unknown, but Google came clean about the change so we know they did.
This change has profound impacts for external link management as part of a search engine optimization campaign. Instead of dropping the links from the equation, as above, they are now included in the equation just not issued their share of the links. So in the above example with 20 points of weight and 20 links, 10 of each type. The 10 regular links now only get 1 point each, and the 10 nofollow links get assigned 1 point, which is simply not delivered to them (the weight evaporates from the system, so to speak). So the regular links got their link weight share cut in half.
Imagine if you will that you have a blog post that was so well written and so popular it got 150 comments, each with a link by the comment submitter. Suppose as well that with your menu and archive and other links on the page you have 50 internal links. So with 200 links total, 150 of which are comments, 75% of your links are external. That means 75% of all the link weight this page ever gets is going to be sent to other sites, sites that could be spam, or bad neighborhoods, and by rewarding comments as such you're only encouraging more spam comments, you're rewarding the spammers. Not only that, you're hurting yourself, by sending so much of your weight away with only 25% being sent to the other content on your site.
If you used nofollow on those links you're no longer encouraging spam comments, but you're still hurting yourself, because now instead of 75% of your weight going to the comment links, it is just evaporating, and you still only have 25% going back to your other content.
In short, using rel=nofollow on user contributed links as a method of link weight retention is no longer valid.
Suppose as in the example above 75% of your weight is going to the comment links. Now suppose that all the comment links must pass through a gatekeeper page, and on this gatekeeper page is your normal site menu, header, and footer, and all the 50 internal links that goes along with it, along with a disclaimer and finally the link to the original comment poster's site.
So 75% of your weight will filter to the gatekeeper page, where there are 51 links. Through the miracle of rounding let me say that 98% of the weight on the gatekeeper page transfers through the 50 links going back to your site, and only 2% of that weight goes on to the original comment link. So, whereas before you were sending 75% of your weight to the comment links, or with nofollow 75% to the abyss, you're now sending a mere 1.5% (2% of 75%) of your weight to those places, the other 98.5% flows back through your site via your menu.
I have had created two plugins, one for Wordpress, one for vBulletin, that will allow you to easily implement two-step external linking. Both let you define the disclaimer that will display on the redirect page. Both let you define whether or not the final external link on the redirect page will be nofollow or not, and both allow for a whitelist for you to set domains that will be exempt from the redirect process.
Finally, because of the simple process through which the redirect works, you can filter any link through the system by hand or by altering your software if you have a custom or other CMS solution you also use.
These plugins are copyrighted, but are free to use and share. You may find them below.Two-Step Linking for vBulletin
Please note to download the vBulletin one you must register at vBulletin.org (but it is free) and to download the Wordpress one you must register at our Website Publisher forums (also free).
You can discuss these plugins either in the forum thread for downloading. Or in the comments of this blog post.