The airbag is located under the seat cushion upholstery, but on top of the plastic seat shell, which is moved by the seat mechanisms. This means that the airbag is always in the right position relative to the occupant.
In an accident, with the occupant in a reclined position on a flat seat cushion, without additional measures there would be a danger of the pelvic area sliding beneath the belt strap (submarining). This can be prevented by the cushionbag, as it raises the front section of the seat cushion upholstery. The airbag is only activated if the seat is in the reclined position during a crash. It is not activated in the upright position, as there is then no danger of submarining. (Source: Daimler AG)
