It rotates when any of the avast resident scanners are scanning a file, when you start a program it would intercept the call to run the .exe file, scan it and the file would run if clean.
When you access a file (hence the name on-access scanner) that would be monitored by avast's resident/on-access scanners, that could be open or run, etc. When browsing a web site avast scans the pages/images, etc. before they are placed in your browser cache so your browser can display them.
Have you done as suggested and enabled the 'Show detailed info on performed actions' option, it you also do it for the Web Shield you would see the list of files scanned by avast. Like the image I have posted here, it might look a little weird as I have the transparency turned up high.