Sonar reports file system activity. It reports all changes. It does not report reading activity.
Sonar cannot prevent disk activity. It only can report it. However, you can use the information reported by Sonar to delete files you don't want on your hard drive.
Sonar requires OS X 10.4, known as Tiger.
Sonar uses the same technology that underlies Tiger's Spotlight feature. However, Sonar does not use Spotlight itself and does not interfere with Spotlight in any way.