The extension spring looks like the old fashion box spring you see in mattresses. They are located at the sides of the door and are mounted on the swing arms. They have a hook on each end and when purchased, they will be in their compressed state. Because they have no protection factors when stretched, it is the more dangerous of the two springs. It is also more inexpensive compared to the torsion spring.
Its main difference from torsion springs is that the operation of the extension spring is based on distance; the further it is extended, the more force is applied on the spring. It can last from 5,000 to 10,000 lifts.