Transferring Cities to Allies

In the FAQ under "The game: advanced", under 4.1 - Diplomacy, it says that "It's also possible to pass empty cities between allies." How do you do that?

One player empties all the units from a city.
His ally can move units to that city.
When the units have been moved to the city, it has been transferred.

1. Empty your city.
2. Tell your ally to take it.
3. Watch your ally taking it.
4. ???
5. Profit!

Thanks.