Once you have configured the custom relationship permissions, you will have the ability to give this custom relationship visibility into private Feedback of the employee they have been set to oversee.
To view this, the person who has been given this visibility and applied this custom relationship can go to the Profile page of the user they now have visibility into. The Profile page can be found under the user's My team tab within the profile navigation.
Here, they will be able to see any Feedback (private + manager and public) this employee has been given.

As a reminder, this gives only visibility. The user will not be able to take action (such as delete) a piece of Feedback.
Create Feedback requests
Dotted line managers can request feedback for any employee they have been assigned visibility into via a custom relationship.