Answer:
The author's use of flashbacks to tell the narrator's story creates tension in the following way:
It makes the reader anticipate, in a tense way, the dreadful household events that had led the narrator to be sentenced to death.
Explanation:
We can see it in the following passage, which can be found at the very beginning of Edgar Allan Poe's classic short story:
"FOR the most wild, yet most homely narrative which I am about to pen, I neither expect nor solicit belief. Mad indeed would I be to expect it, in a case where my very senses reject their own evidence. Yet, mad am I not -- and very surely do I not dream. But tomorrow I die, and today I would unburthen my soul. My immediate purpose is to place before the world, plainly, succinctly, and without comment, a series of mere household events."
Poe's use of flashbacks is there for a good reason, and it surely achieves its purpose, since the story ends with the reader finding out the narrator had killed his own wife and the black cat had given away where her corpse had been hidden, therefore helping the policemen who were at the narrator's house in order to interrogate him.
