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