Active bolster with active venting

An active bolster for an automotive vehicle includes a bladder that inflates an interior trim area of the vehicle to provide a cushion to an occupant during a crash. The bladder has a front panel and a back panel joined to the front panel defining an inflatable cavity. The back panel has a variable vent for venting inflation gas supplied into the cavity during a crash event. The vent has a first predetermined flow coefficient when a pressure difference across the vent is substantially zero and a greater flow coefficient when there is a predetermined pressure difference across the vent. Little gas is lost during an inflation phase while sufficient venting is obtained during an impact phase.