This plugin allows you to crosspost your entries to both your wordpress and mixi at the same time. Mixi is one of the most popular SNS sites in Japan.
Once you install this plugin, you will see the option in the edit screen. If you want to publish your post to mixi, you can simply check the option and publish.
You can find the Japanese description at here.
此外掛程式已可供下載，並可用於你的 WordPress 自助託管安裝。