What does the night train rule mean?

If you have an Eurail pass and are traveling on a sleeper train, only the day of departure is counted as a travel day.

For example: you take a train from London to Amsterdam on the afternoon of the 31st of August. In Amsterdam, you change for the sleeper train to Zürich, departing on the 31st at 18:00 and arriving the next morning at 09:00 on the first of September in Zurich. Only the 31st counts as a travel day on your Eurail pass; only one of your travel days has been used. 

However, if you then take a train from Zurich to Geneva on the 1st of September, that day will count as a second travel day.