Away time for restarting duration of stay period

How long must I be out of China to restart my duration day count when I re-enter? Can I just fly to Seoul for a day and then back into china and have a new duration period start? Or do I have to return to the USA and wait for a certain amount of time before going back to China? I have 60 day stay for 12 months with multiple entry.

Also does going to hong kong count as leaving the country?