I never understood tht until I heard the following: Souls in Gehinom get a reprieve for Shabbat. Their reprieve lasts until the last shul says havdalah.
Shabbos basically lasts 48 hours somewhere on the globe, If you start Shabbos in New Zealand which is about 20 hours ahead of NY and end Shabbos in Hawaii which is 5 hours past NY the total time is about 48 hours