Most residential garage doors have one torsion spring. Some are equipped with two, but this is a less common setting. The spring is mounted over a metal shaft which is right above the door. There is a drum at each end of the shaft. The lift cables go through it. During installation, the spring is wound to get enough tension to counterbalance the door. When the door is opening, the spring unwinds and the tension is released. It is applied to the drums and then to the cables so that it can be used to open the door. When the door closes, the spring winds with energy once again.
These always come in a pair of two when they are used in residential overhead doors. They are installed above the horizontal tracks. On one side, they are held in place by a bolt. On the other, they are connected to a pulley with the help of a fork. When the door is closed, these springs are stretched. The energy which they hold in this position works to counterbalance the weight of the door. During opening, the springs contract and release the energy. It is passed to the lift cables via a system of pulleys. All extension springs have safety cables running through them. These are set during installation. Their job is to keep the pieces of the spring in place when it gets broken. Otherwise, they can snap out and hurt someone or something nearby.