What technique does Conrad start using once Marlow begins to speak?

When Marlow begins to speak, we realize that the story is being told as a story within a story. We have two narrators: the first, an unnamed sailor which is aboard the Nellie with Marlow, and the second, Marlow himself.
This type of technique is called a frame narrative. Many authors have used this technique, as it provides a shift of perspective and allows the reader to gain a better perception and understanding of the story. The first narrator is also used as means of setting up the themes of the story without diving directly into the narrative. The first narrator also takes on our position as audience, whereas Marlow becomes the primary narrator later on.



Marlow himself is also a narrative device which allows Conrad to tell the story from a different perspective. Through Marlow, Conrad recounts events and observes and analyzes the other characters. Marlow is attempting to tell Kurtz's story just as much as his own. He is an unreliable first person narrator, as he is unable to fully convey Kurtz's inner state of mind and can't really experience the darkness which Kurtz has experienced. Marlow is not trying to hide anything or purposefully lie; more simply, he does not understand the meaning of what he is saying at the beginning of his narration, and it only becomes clearer as his story continues. He continuously shifts between present and past in order to have a clearer understanding of what happened but fails to grasp the real darkness which Kurtz experiences.