This simple animation shows as the dampers close to each zone, one at a time, the bypass should gradually open until... when only one zone is left open, the bypass is open all the way.  Airflow can be seen going back through the system to relieve the pressure in the duct.  The purpose of bypass (pressure relief) is to minimize air noise at the register (air vent in the home) and to prevent over pressuring the duct system which can have many negative implications including not enough airflow.