Physical Harm from an Expelliarmus

[erisedstraeh2002]

No: HPFGUIDX 44175

In griping about how the next Movie That Must Not Be Named appears to 
be handling the duelling club scene in CoS, a thought occurred to me -
the Expelliarmus spell is used to disarm an opponent, yet when Snape 
casts an Expelliarmus on Lockhart in the duelling club, "Lockhart was 
blasted off his feet: he flew backwards off the stage, smashed into 
the wall and slid down it to sprawl on the floor" (Ch. 11, p. 142 UK. 
ed.).  IIRC, the only other time an Expelliarmus does something other 
than disarm, it's when HRH cast it simultaneously at Snape in the 
shrieking shack and "Snape was lifted off his feet and slammed into 
the wall, then slid down it to the floor, a trickle of blood oozing 
from under his hair.  He had been knocked out" (Ch. 19, p. 265 UK 
ed.).  

When Sirius croaks "Expelliarmus" in the shrieking shack in PoA, 
Harry and Hermione's wands fly out of their hands, but no physical 
harm is done to them.  Same goes for when Lupin yells "Expelliarmus" 
in the shrieking shack in PoA -  HRH's wands fly out of their hands, 
but  again, no physical harm is done to them.  When Harry 
yells "Expelliarmus" in CoS, the diary flies out of Draco's hand, but 
again, no physical harm to Draco.  

I assume the Snape knock-out was a result of the combined force of 
three Expelliarmus-es thrown simultaneously.  But what about when 
Snape blasts Lockhart in the duelling club?  Is this a sign of 
Snape's power?  Or of Lockhart's weakness?  Or both?  Or does the 
physical harm happen when you throw the Expelliarmus in a rage 
(since "Snape was looking murderous" when he threw it)?  Or can a 
wizard cast an Expelliarmus with different intentions – to merely 
disarm, or to both disarm and cause physical harm?  

~Phyllis
who wonders if JKR got the idea for the Anglia's Invisibility Booster 
from the invisibility button on Stuart Little's model car