For many years people filed or ground mainsprings to lighten the tension even though experts said not to do it because the spring would sooner or later snap.
They did it anyway and were often shocked when the spring snapped soon after.

What happens is, the filing or grinding leaves scratches, often microscopic and those cause stress risers in the structure of the spring. That causes the spring to snap.
People were warned that any filing or grinding required extensive smoothing of the spring to remove even scratches that couldn't be seen to eliminate the stress risers, but even then the springs still tend to snap.

Again, with all the different types and brands of S&W mainsprings available, bending or grinding of springs is a relic of a different era before after market springs were available.