Exactly what does it mean when "virtual" is within "class Foo : public virtual Bar" instead of "virtual void frob()"?

For any given method you will find 8 cases stemming in the presence or lack of virtual within the following three locations.

  1. A superclass's functions.
  2. The inheritance chain with this class.
  3. This classes functions.

I believe I realize how amounts 1 and three interact but number two appears redundant. Could it be? What am I lacking the knowledge of?

That's virtual inheritance, you're doing so when you are aware you will be doing multiple inheritance. That page adopts a lot more detail.