The "Select Elements" tool works like this:
- Choose a type of elements you want to select. (You do that by checking the appropriate "Use These Criteria" checkbox.)
- Iterate over the elements of the type chosen in step 1.
- Collect each element that satisfies the search criteria.
"Act On Current Selection Only" restricts step 2 above from "iterate over all elements" to "iterate over the selected elements of the type chosen in step 1".
I.e. you were telling the tool to collect a subset of the already selected edges. However, there were no selected edges in your case (because it is not possible to have selected nodes and selected edges at the same).
"Act On Current Selection Only" can be used for one purpose only: To further reduce the set of currently selected elements through an additional search criterion for elements of the same type. And to make that use case work, you will always need to check "Clear Selection First" as well.
E.g suppose you want to select all red nodes labeled "select me". Since the tool's UI does not offer a way to specify several search criteria, you would have to select all red nodes first, then run the tool with "Act On Current Selection Only" and "Clear Selection First" for nodes labeled "select me".
