How is it that Wormtail and others can use other's wands?

[fuzz876i]

No: HPFGUIDX 147756

I have recently reread The Goblet of Fire and The Prisoner of Azkaban 
and both books raised some questions.  In the Sorcerer's Stone when 
Harry buys his wand he is told the wand chooses the wizard. In the 2 
books mentioned other people used some one else's wand.

In Prisoner of Azkaban Sirius used Ron's wand to disarm Harry and 
Hermione. He used Snape's wand to magic Snape out of the Shrieking 
Shack. He also used Snape's wand to make Wormtail show himself as a 
human and not a rat. 

In The Goblet of Fire Wormtail used Lord Voldemort's wand twice to 
cast the Avada Kedavra curse once on Frank Bryce and once on Cedric 
Diggory.

What I want to know is this:  how is it possible to use someone 
else's wand when that wand is supposed to be specific to the owner 
and backfire if someone else uses it?

fuzz876i