You may use this feature to track if a campaign is functional and working as expected (e.g if mails arrive, the landing page works as expected etc.). With this feature, the system will periodically send emails to a test address, while the campaign is running. That will help you to make sure that the attack is executing correctly.
The email tracking can be configured in the base settings (first screen after creating a new campaign). You can enable the checkbox and enter the mail address, where you want the tracking mail to be sent. The tracking interval (in emails) specifies the how many tracking emails are sent. For example, if you specify 10 , a tracking email will be sent after every 10 email delivered to the client. If you have a campaign with 1000 users, you would then get 100 tracking emails.
The tracking email and all the clicks generated from a tracking email are not logged in the system and therefore will not affect the campaign statistics.