In addition to protecting drivers’ or passengers’ heads during a side-impact crash, some side-impact head air bags, or "curtains," can also protect occupants from injury and ejection during a rollover crash. This is important because ejection causes most injuries and fatalities in rollover crashes - most people who are killed are not wearing safety belts to hold them in place.
Not all side-impact head air bags are designed to deploy as rollover air bags. Check with your dealer and vehicle manufacturer for the availability of side-impact head air bags that can also operate as rollover air bags.

HOW ROLL OVER AIRBAGS WORK?
If a rollover is detected, the side-impact head air bags are typically triggered in combination with safety belt retractors to remove slack from the safety belt and keep the occupant firmly in the seat. Most side-impact head air bags deploy downward from the overhead roof rail, very close to the side windows. In many cases the rollover sensing system can determine an imminent rollover when the roll angle is very small and all four wheels are still on the ground.

When deployed as rollover air bags, side-impact head air bags will stay inflated longer to help protect the heads of the occupants during the rollover. They also keep the occupants of the outboard seats from being thrown from the vehicle. The combination of these air bags and properly worn safety belts can significantly reduce the chance of ejection.