How do skin air sacs protect plunge-diving boobies and tropic birds?

For boobies and tropic birds, the system of air sacs located under the skin serves as a protective mechanism during their high-speed plunge-dives into the water.1

These air sacs are connected to the respiratory system and form a spongy, cushion-like mattress. This structure is believed to protect the bird by absorbing the force of the impact when it strikes the water's surface at high velocity. In addition to this protective function, these air sacs may provide the birds with advantageous buoyancy. In some species, such as the blue-footed booby, specialized air sacs in the skull also help protect the bird from the physical pressures encountered when swimming at significant depths below the surface.12