The spring pushes a little plunger that keeps the mag cap from unscrewing. To remove the cap you need to depress the plunger on the side of the mag tube then hold it down to begin turning the cap. There is a spring under pressure in the magazine tube so be ready for it as you turn the cap. The new spring lives under the plunger and will need to be compressed a bit to re-install. Not too bad of a job,just do not try to turn the cap without depressing the plunger with a small punch or similar device.