In a narrative, the rising action comes immediately after the exposition and serves to create suspense in the plot, to trigger the events that will lead to the climax.
In "The Adventure of the Cardboard Box," the rising action is when Sherlock Holmes and Dr Watson meet with Inspector Lestrade from Scotland Yard and Miss Susan Cushing in Croydon.
Miss Cushing had received a yellow cardboard box the mail, which contains the ears of two different people, buried in salt. Cushing does not have any idea who would sent this to her or why.
This creates tension in the story and sets forth the following twists and turns, because Sherlock Holmes gets a first-hand look at the box and the ears for the first time, thus triggering his involvement in the case.