The principal of operation is very simple.  If you imagine taking a rollup window shade and mounting it upside-down and fixing the loose end to the top of the door opening.  Then as the roll/tube rolls up and down the inside of the building, the door either opens or closes. 

When the roll is all of the way down and touching the floor seal the door is closed.  As you open the door by powering the motor(s), the door starts to open, starting at the floor and rolling up the wall until it is fully open.

The door can be stopped at any point along its travel.  This is convenient to: 1) allow for some ventilation, 2) to raise just enough to allow personnel or smaller equipment to move in and out or 3) raise just enough to allow smaller aircraft to pass while trapping and conserving the heat at the top of the building.