Clearly every good piece of fiction will contain multiple examples of different writing techniques, and the work of Chopin is certainly no exception. There are plenty you can pick from, and what is great is that because this story is so short, you can re-read the story very quickly and identify the variety of techniques there are included.
For me, however, the most powerful technique is the use of irony. Notice how this irony can be found in the very first and last lines of the story:
Knowing that Mrs. Mallard was afflicted with a heart trouble, great care was taken to break to her as gently as possible the news of her husband's death.
When the doctors came they said she had died of heart disease - of joy that kills.
However, we as readers know the internal thoughts of Mrs. Mallard and how, once she accepted her husband's death, the overwhelming thought that dominated her mind was her freedom now that she was alone:
There would be no one to live for her during those coming years; she would live for herself. There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature.
Thus the delight of this thought and her freedom is cut short dramatically and suddenly by the re-appearance of her supposedly dead husband which results in Mrs. Mallard's death. Of course, we know it was not of "joy that kills," it was the thought of being placed suddenly back into that relationship again that caused her death after being able to contemplate freedom.