Normally the driver side airbag is folded into the center part of the steering wheel and is stored inside the steering wheel cover. When a collision occurs, the airbag breaks through the tear-seam (an indented line partition) on the back side of the steering wheel cover as it inflates to protect the driver.
Driver Airbag Deployment Videos
Slow Motion Deployment of Actual Airbag
Slow Motion in-car Simulation