Here's how I think of it: It's the worst of both states.

Feeling seriously depressed thoughts but you can't sit still and your head is racing.
Anxious, racing mind, but all you can do is lay in bed.
Suicidal - thinking there's no way out.

Whenever I'm having a mood that doesn't fit nicely into my depressive or hypo categories and is godawful, it usually means a mixed state.