Carol,

Rather than going into some kind of metaphysical and nuanced explanation that’s as easy to hold as a soup sandwich...  Why not talk in more concrete terms that are a bit easier to grasp and upon which we can all agree?  For starters, please consult the origin of the term "Shadow" as coined by Bob Kipple.  It's defined in "the kip scale” link on the left:

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In simple terms as Bob defined them...  Shadows are the pain of cluster headache attacks ranging from Kip-1 through Kip-5.

I prefer to use a more defined physical scale we can all understand like water freezes at 32º F (0º C) and it boils at 212º F (100º C).  If you compare this to the 10-point pain scale, it's Kip-0 to Kip-10.  Now find a point in the middle we can agree upon like...  When does your eye start to water and your nose run?  I anchor that clearly recognizable cluster headache symptom at Kip-6 to Kip-7.   How's that so far?

That's still not sufficient to accurately calibrate the pain scale...  so here's another recognizable cluster headache symptom I think most of us can also agree upon and that's the pain level that wakes you up and at which you cannot get back to sleep.  I've found I wake up around Kip-4 and cannot get back to sleep at Kip-5 or above...  that is…  unless I've had too many rum & cokes.

Now...  just to toss a little more bacon on the fire...  I'll make another observation...   I think there are two types of cluster headache attacks that I call "shadows" and "stingers."  In a numerical sense, and as explained earlier, shadows are cluster headache attacks where the pain is primarily vascular in origin and they fall in the range of Kip-1 to Kip-5.  Stingers cover the range Kip-6 through Kip-9.  They include the vascular pain and also involve the trigeminal nerve with progressively more extreme pain and most if not all the classic cluster headache symptoms…  
 
I stop at Kip-9 as I agree with Chuck…  A Kip-10 is a very special kind of cluster headache that only a few ever experience.  I’ll further qualify that by saying a Kip-10 in Star Trek terms is like going into Warp Drive…  A lot of people claim to have regular attacks at Kip-10, but after talking to folks like Chuck who have actually experienced them over the years, there is nothing regular about them and you'll know when you've had one.  It’s a cluster headache like no other with pain so severe and so excruciating, the ER is the only solution.  And when they finally end... you're left so profoundly washed out and wasted, sleep is the only thing that happens next.

So shadows are a lot like shit…  they just happen...  They creep up slowly, last from twenty minutes to as much as a day or longer, then fade away.  They annoy you rather than hurt at the lower Kip-levels, and rarely ever escalate beyond a Kip-5.  When they do get that high, they consume your attention with a gnawing pain that makes other activities impossible.  

Stingers, on the other hand, let you know at the onset of an attack, that the Space Shuttle engines are at 105% thrust, and both booster rockets have just ignited.  Lift off is inevitable.  They frequently escalate from onset to Kip-7 or Kip 8 in less than two minutes... much less time for some... and you leap or run for your abortive of choice.  You move even faster when a stinger wakes you from a sound sleep because you know the beast already has a head start making an abort long and difficult.

Of course these are just my observations, and I know we are all wired differently, but I hope this helps answer your question about shadows.  I treat shadows like any other cluster headache and I abort them with oxygen therapy at flow rates high enough to support hyperventilation as fast as possible.

Take Care,

V/R, Batch