How does Conan Doyle make the end of "How It Happened" so effective?

Doyle’s ending is effective for two reasons. The first is
that clues are given to guide us to the conclusion. The second is that he constructs the
ending in such a way that an alternate ending is plausible. The first clue frames the
whole narrative an comprises the two opening lines: "She was a writing medium. This is
what she wrote."

If you know what a medium is, then you know that the
speaker--a narrator who is different from the first-person narrator who speaks
throughout the rest of the story--is introducing a tale of spiritualism--a narrative
account told by a spirit. If you don't know what a medium is, or don't connect it with a
"writing medium," then you are kept guessing right till the
end.

Another clue is the sudden, unexpected appearance of the
narrator's favorite college friend with a peaceful smile. There is even an embedded clue
within the introduction of this new character because the narrator says that he was
willing to take things as he found them, which confirms the unusual nature of Stanley's
visit:

I was
surprised to see him , but I was like a man in a dream, giddy and shaken and quite
prepared to take things as I found them without questioning
them.

These clues combine
with the details about the master's location after the wreck. Doyle does something a
little unusual and throws the master's spiritual body away from his physical body. This
allows for the introduction of Stanley while both are surrounded by peace and calm. This
twist to the usual ghost story allows us the opportunity of anticipating an alternate
ending in which the narrator survives the crash--that is it's possible until we hear
about the "hushed" voices.