Based on an early postach.io blog - http://blog.postach.io/tumblr-import-twitter-posting-ui-updates-speed-increases

Now you can tag a note as "tweet" and it will post to your Twitter, although you need to link to Twitter first..

Auto-tweet: You can enable it in the "Edit" section of your site.

Once in "Edit" choose Imports & Sharing from the left menu.

Then simply click the "Connect Twitter" button to hook up your Twitter account. You can connect one Twitter account per site you run on Postach.io.

IMPORTANT

If you tick this box, then a notification will go to Twitter every time you post, and not only when the 'tweet' tag is set.