You don't know which end is up. Things may not get much better when the slide stops. Even if you're still alive, it's no joke being buried in a mass of snow that resembles concrete more than fluff. Even if your throat isn't plugged with snow, the snow may compress your chest and stop your breathing. If, as instructed, you manage to carve out a little air space around your face as the slide grinds to a halt, heat from your breath will soon ice up that lifesaving air cavity.
Asphyxiation is the major cause of death in avalanches.
Fewer than half of those who are totally buried survive, and nobody who's been buried deeper than seven feet has lived to tell about it. A loose snow avalanche starts at a point on the slope and widens as it travels, releasing more snow. As the slab slides, it breaks into great chunks of hardened snow that trap or injure victims. After a slab avalanche, a clear fracture line usually marks the point where the slab cleaved away from the slope. Traditionally, the victims have included skiers and climbers.