Question

How does a buffer resist change in ph upon addition of a strong acid?

Answer 1

Any buffer exists in this equilibrium
 
  HA <=> $H^+ + A^-$
In a buffer, there is a large reservoir of both the undissociated acid (HA) and its conjugate base ($A^-$) 

When a strong acid is added, it reacts with the large reservoir of the conjugate base ($A^-$) forming a salt and water. Since this large reservoir of the conjugate base is used, the ph does not alter drastically, but instead resist the pH change.