When a mainspringMainspring The mainspring is a spiral-shaped metal spring that stores energy in a mechanical watch movement. It is located inside the barrel. The mainspring is wound, either manually or automatically. The mainspring unwinds over time, and the watch will stop running when the mainspring is fully unwound. [Learn More] is fully wound, the power it generates will (by physical fact) be higher than when the spring is nearly unwound. As such, your watch gradually loses power, in a downward sloping curve. This affects the overall accuracy of your wristwatch. The Fusée and ChainFusée and Chain When a mainspring is fully wound, the power it generates will (by physical fact) be higher than when the spring is nearly unwound. As such, your watch gradually loses power, in a downward sloping curve. This affects the overall accuracy of your wristwatch. The Fusée and Chain mechanism exists for the sole purpose of negating that effect to the maximum extent. Its purpose in life is to avoid an uneven energy release as the mainspring unwinds and ensure that the movement receives a constant amount of energy. No downward curve here, but a straight line that comes to a hard stop when the power runs out. [Detailed Article] mechanism exists for the sole purpose of negating that effect to the maximum extent. Its purpose in life is to avoid an uneven energy release as the mainspring unwinds and ensure that the movement receives a constant amount of energy. No downward curve here, but a straight line that comes to a hard stop when the power runs out.