If you have the module on a page of your site like the home page, or a page with a menu, then people may find the module and may enter a link.

If however you send them an email, saying... "Hay, lets swap back links" and invite them to the URL with the module on it, you are more likely to get people come and submit links.

The trigger role will send an email message (that you define in the module) to anyone added to that specific role or roles. So if you want to invite all users who register on your site to swap links, use the registered user role (like we have). 

But if you want to manually define who is sent the email invitation, then create a new role (call it link builder or something) and manually add users to that role. When you add someone to that trigger role, they will get the welcome message from the module.

Clear?

Cool.