Expelliarmus and backfiring

[happyjoeysmiley]

No: HPFGUIDX 184544

Okay, I'm sorry if this has already been discussed.

When Voldy used the AK curse when Harry was a baby, it backfired - 
okay, Lily's sacrifice acted as an invisible shield and repelled the 
curse, I guess. Fine.

When Voldy used the AK curse in DH climax, Lily's protection was not 
with Harry. Harry is the master of the Elder Wand and so the curse did 
not kill him. OK. Now, Expelliarmus spell only disarms the opponent. 
So, how did the AK curse *backfire* on Voldy? I think backfiring would 
have made sense had Harry used Protego. Not sure how Expelliarmus 
would help with respect to backfiring.

Any thoughts / quotes from canon?

Cheers,
~Joey, who always smiles to herself whenever she thinks that JKR made 
it a point that Harry learnt the Expelliarmus spell from *Snape* :)