I'm sorry, but flicking CAN be as bad as hitting as can shaking.  Try to
get a chance to visit an anatomy dept.  at a med school sometime then go to
the gross anatomy section and ask to see an exploded skull.  These are very
hard to make, BTW.  You need an individual who was fully grown but young
enough that the skull sutures are not tightly woven since they become more
complex over time.  Then the brain has to be safely removed at least in
large part and legumes added to the brain case and very, very swollen over
a long period of time to gradually separate the sections.  When you look at
the skull note a few things.
 
In relation to shaking note that the brain kind of floats around so it
moves when there are major jarring situations (not a surprise to those who
have read about deaths or damage form shaken baby syndrome, or even of the
recent compilation of surface injuries to the brain from regular roller
coaster use); notice that there are some very rough features inside a
skull.  Imagine a soft brain being dragged over those.  You get the idea.
 
In relation to nose-flicks ask to see the fine bones within the sinus
cavities.  Note that even within humans these are are paper thin.  Remember
reading you may have done in relation to battering cases in which people
had to later have sinus surgery to repair overgrowth of boney tissue or
scar tissue in response to injuries to such delicate sites.  Heck, I had
one myself, and my sister the other.  These are delicate structures.  Just
because you can't see them on the outside doesn't mean that they are not in
need of consideration.
 
When you aren't sure what constitutes abuse recognize it as a health issue
and don't ask someone who doesn't know the animals' anatomy; instead ask a
vet who is knowledgeable for that species, same as you would for any other
physical aspect of health and well-being.
[Posted in FML issue 3062]