More specifically it's the bright light triggering the iris to close that improves the focus. As the aperture narrows (iris closes) the incoming rays become more collimated (less scattered) and so the image falling on your cornea appears sharper even if your eye's lens is not able to focus it correctly. At it's extreme this effect allows the pinhole camera to work without using any lenses at all to focus the image.

The thing is though, while using lower powered glasses might help you train the muscles around your eyes' lenses to be stronger, mitigating the myopia slightly, the root problem still remains: your eyeball is the wrong shape and there's only so much your eyes' lenses can do to make up for that.