Why is a cellar safe during a tornado

We were staying with some relatives in tornado alley this weekend, and they had a cellar (small underground room). My aunt mentioned that the cellar saved her life during a tornado a year or so back. How? Why would an underground room be safe? Wouldn't that create a wind tunnel, if say the doors were pulled open? (a tornado can pick up a car, I don't think some iron doors would stop it). Also, why would lying in a ditch be safe? Wouldn't it just...pluck you up?

The wind can't pick anything up off the ground unless it is higher than the surface of the ground. Most of the stuff that lays flat on the ground doesn't get picked up by the wind such as dust until something else bumps against it and gets it to bounce up off the ground. In deserts, sand storms can't pick up a grain of sand until other particles hit or roll across the surface causing the other grains of sand to roll and bounce along the surface.

In a basement, the wind loses its force when it leaves the main vortex such as when the winds drop into a hole. Think of wind like water. As the water flows across the stream bed it moves fast and with force, but if the bottom of the river drops into a hole, the mechanical force of the river isn't behind the eddie that drops into the hole. An arrow has penetrating force when shot from a bow because it has the weight and strength of the shaft behind it.

Wind by itself can't pick you up without the millions of tons of air behind it pushing it. As far as sucking you out of a cellar, if you pay attention to physics, there is no such thing as suction. It's just a displacement of air. The air inside the room rushing to fill the air outside the room. When you suck on a straw, you are pulling the air in the straw into your inflated lungs or a sudden void needing to be filled. The air being drawn in must be replaced with something so its the fluid in the glass filling the void.