Clipstart treats Twitter as just another upload service. When you upload to Twitter, Clipstart uploads the video file to Yfrog, which then posts a URL to the uploaded file to your Twitter account for all your followers to see.
If you click the "Upload to Twitter" button, Clipstart will prompt for your Twitter username and password and then send the video through Yfrog, using the video title as the tweet message body. If you want to specify a custom tweet message, use the Video -> "Upload with Options" menu command and enter the text you want to tweet in the description field.