Garage door torsion springs are an important part of a door because the mechanism will not be complete without it. You may think that it is easy to replace it once broken but you better think again. It is very complicated and dangerous to replace, especially if you are not an expert in garage door repair. This is mounted on top of the door and together with the shaft it will lift drums, which wind up the cables at the bottom and lift the door.
This little but very complex object usually breaks due to metal fatigue or turns rusty after a few years of use. The one that is commonly used is made with galvanized steel so it can last longer. It is preloaded with a set of torsion bars that are locked into torsion tube via screws. A rotational potential energy is being built inside the tube to make it work. The energy is increased by a pulley and distributed through the garage door cables on either side of the door. The cables will have tension and it is equal to the weight of the door in any position. That mechanism will make the garage door feel weightless. Load will be applied to the spring and it will cause the spring diameter to decrease and increase the length. There is a maximum load that a spring can take and that is maintained so that no parts will be broken.
The physics behind that simple part is complicated. That is why garage door maintenance should not be taken for granted. A misaligned track or a worn-out cable can affect the process within the torsion springs and it will lead to more broken parts. So, remember to inspect everything regularly for your own safety.