Anecdotally it seems that people are especially vulnerable to diagnostic overshadowing when they have received a diagnosis of borderline personality disorder. Because clinicians tend to associate this diagnosis with the classical idea of "hysteria" - the supposed eruption of emotional distress into the realm of the physical symptom - physical complaints or apparently neurological signs are apt to be considered psychosomatic. Thus a person with this diagnosis may have clear medical causes of physical pains that fail to get discovered.
But of course even without a depression in the picture, deep feelings of sadness, grief and despair are a part of our lives. We accept this and we live through our sorrows. They teach us about who we are and what our life is. Without a diagnosis of depression, our experience of such feelings is seen as part of the mix of ourselves and our context.
Diagnostic underwriting would occur where a depressed person's ordinary feelings of misery are mistakenly attributed to their depression; chalked up to a disorder that appears to account for things that it can't.
It might look like this: You feel hopeless all of a sudden, or guilty. You would have done regardless of diagnosis; it was something you experienced, thought or did that made it so. But because you have the diagnosis on hand, you don't understand it as a part of yourself but as a part of something else that has attached itself to you. You have attributed part of your experience to a phenomenon it doesn't belong to. Diagnostic underwriting has occurred.
Note too that diagnostic underwriting might be imaginable in theory but impossible to discern in reality. Who can say what really is me and what is really is my disorder? Who can really discern between ordinary and pathological feelings? In any case aren't these false distinctions? There is no safe place to stand in teasing this out, but the idea would be to talk about what happens as you claw your way out from under the emotional cloud of an alien experience.
Scared to feel and to trust what they feel, the individual recovering from a mood disorder has a twinge of emotion: "is this sadness OK, or is it going to be the start of my fall back into depression?" The answer may be practically unknowable, but the question is still an important one to grapple with.