72 hours visit to china with no visa required, how does it work? 72 hours when does it begin?

Generally speaking, 72 hours begins from the scheduled time of your flight arrival.

If your flights' schedules are qualifed, you need to tell the airline company's staff that you want enjoy this policy before you boarding. Upon reaching either of the city, you just go the 72-hour free transit special line at the immigration in the airport to get a stay-permit stamp on your passport. You still need to show your onward tickets when passing the immigration. After getting the stamp, you can get out of the airport.