Extension springs are long, lighter-weight springs that run perpendicular to the door and are mounted above the horizontal portions of the door tracks. These springs are tensioned by stretching out, using cables and pulleys, as with the torsion system. Because extension springs are merely suspended between two brackets (they are not mounted to a rod, like torsion springs), they must have a safety cable running through each spring. This helps contain the spring in the event of a breakage. Without the safety cable, a spring breaking under tension is a very serious safety hazard. If you have old springs that don't have safety cables, you should install them even if you aren't replacing the springs. https://m.youtube.com/v/Z_eZc-kh40c
Aging garage door springs cause the door to effectively "weigh" more as the steel loses its resiliency. With new springs, a heavy garage door should take no more than about 10 pounds of force to lift into an open position. With springs nearing the end of their lifespan, the force required to lift the door can be considerably more, since a garage door may weigh 200 pounds or more.

10.1 It is now time to wind the new springs, but before doing so, I recommend marking the shaft just beyond the winding cone. This is a final step taken to assure that you have installed the springs on the correct sides of the center bracket. Torsion springs always grow in length when they are wound in the proper direction. If your spring does not get longer as you wind it, you are winding it the wrong direction probably because it is improperly installed. We recurrently get calls about springs coming loose from the cones at about 6 turns. If this happens, switch the springs.