A very complete article has been published on the website “the Naked Watchmaker” about Géneréle Ressorts and its activity:
Mainsprings, their function and how they are made.
Isochronism is the primary goal of every modern timekeeper. It is the characteristic of generating a constant, uniform period of vibration, to be able to assure a consistent rate of time. Although every part of a movement is important to achieve this, much of what can be made following the mainspring and barrel would be rendered ineffective if the mainspring is not well made.
Every mechanical watch requires a power source to drive the wheels that lead to the regulatory organ, which is in 99.9% of watch movements an oscillating balance wheel. The power source is a mainspring, wound either manually by hand through a winding crown, or by an automatic winding system incorporated into the watch movement. Although there are always exceptions to every rule in horology, there exist two fundamental types of mainspring. Those that when fully wound are blocked from turning, made for manually wound movements, and automatic mainsprings. Automatic mainsprings have a longer bridal fixed at their end which push against the inside of the last turn of the spring, approximately 20% thicker than the rest of the spring. This long bridal allows the mainspring to slide on the inside of the barrel (where the mainspring lives) once the spring is fully wound. This allows the automatic mechanism to continue to turn, continuing to wind the fully wound spring without causing any damage.