OK, I finally got a case that shows it does something and here is how to do that (I can provide a video, if necessary).

Step 1 - start Cinelarra and just wipe an area to drag the "Title" plugin to it and type in some worlds to see in Compositor.

Step 2 - now drag the "Blur" plugin to that same area and bring up the menu; click the "Reset" button.  Look in the Compositor to see what it looks like and then Check the "Alpha determines radius" box and you can clearly see a difference.