Some usernames on twitter are not active so Twitter management will release it after some time but they don't annouce when or some users are changing there usernames so it will be available to be registered by other user .
This App will enable you to keep an eye on Twitter to notify you once your desired username is available, Just add it to the watch list and keep the rest for the App.

For Testing :
1- register a username on Twitter
2- Try to check it on the App
it will give you message that "Sorry , This username is not available do you want to add it to the watch list ?
3- Tap on "Yes"
4- then login to the twitter account you created and 5- change the username from Settings > Profile
6- wait for the notification from the App :)