An air bag (12) includes at least one tether (44) and an inflation fluid source (14) having two modes of operation. In the first mode of operation, the inflation fluid source (14) is activated to provide a first inflation fluid pressure in the air bag (12). In the second mode of operation, the inflation fluid source (14) is activated to provide a second inflation fluid pressure in the air bag (12) higher than the first pressure. The tether (44), which helps to control the shape of the air bag (12), tears in response to the second inflation fluid pressure in the air bag.