Exercise 3: Silence Monitoring for a Job That Finishes — Possible Solution
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Steps:

  sleep 30; echo done          # in window 0
  Ctrl-a _                     # start monitoring window 0 for silence
  Ctrl-a c                     # move to a new window and wait

What happens: after "done" is printed, window 0 produces no more
output. Once it has been silent for the monitoring period (30 seconds
by default, though this can be changed), screen shows a message
telling you that window 0 has gone quiet, and marks the window in the
window list.

Why silence monitoring suits long jobs:

A long-running job, such as a build or an import, usually prints
output steadily while it works. Activity monitoring (Ctrl-a M) would
keep telling you "something happened" the whole time, which isn't
useful. Silence is the interesting event: output stopping usually
means the job has finished — or has hung — and it's time to look.

WHY THIS WORKS AS AN ANSWER
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It explains when each kind of monitoring is useful, rather than only
describing the key press. The exact wording of screen's message varies
by version.