The narrative mode is called a frame technique because it's a story within a story. Marqi is correct in that one of the guests on board the Nellie begins the novel; he describes the others on the ship and introduces us to Marlow, who is the primary narrator of the story. Most of Heart of Darkness, then, is told from his perspective as he recounts his experiences to his friends. Several times in the course of his narrative, he pauses or is interrupted by someone, just to remind the reader that Marlow has an audience. The novel ends with the same narrator that opens it, thus "framing" Marlow's story.
Something I noticed is that the first 'I' in the book, as in someone telling the story in the first person, comes from one of Marlow's shipmates in the very beginning, telling us about how Marlow's telling them this story. This character warns the reader to be perceptive of the ebb and flow of Marlow's narrative.
The narrator then shifts to Marlow, as he recounts his experiences. His way of telling the story is through a mix of emotions, events, thoughts, and reactions. Also notice all the paragraphs of his story are in quotations, as if he is speaking to the sailors who are listening to him.
That's all I got, hope it helps :)