The testing button allows you to preview how the e-mail will appear before sending it to the audience.
Click on the "Testing" button.
Select the template you want to use.
Fill in your e-mail. The e-mail will be sent to your e-mail address to show you how it looks like.
Fill in the ID the audience you want this to be sent to.
Fill in the URL address.
To send the testing e-mail, click on the button "Send".