Things with the Pinboard private tag ".twitter" get crossposted to Twitter. Righteous, dude.
On Pinboard, tags that start with periods are considered private tags; this is useful for metatags like ".twitter", where the content of the link you are crossposting have nothing to do with Twitter. Using the . in front helps keep the "twitter" tag on Pinboard full of stories *about* Twitter, rather than stories you happen to want to crosspost *to* Twitter.