Many different terminal devices or "Hooks" Are used to ensure that the spring is being stretched. The extension spring works in opposition to the compression spring. A compression spring acts in reverse when pressed, and a extension spring ATCS in reverse when stretched or pulled. When the ends of the extension spring are pulled apart, the spring tries to pull them back together. Like compression springs, extension springs also absorb and store energy. Unlike compression springs, however, most extension springs are usually under some degree of tension, even without any load. This initial tension determines how tightly the extension spring is wound without any load.