This extension was originally planned to be a Ruby Extension example but turned out to also be a rather useful little extension.
It is used to shrink the selection to a subset of the selected entities using a slider. One practical use case is to randomly select a subset of similar components to replace with another component. If you for instance have numerous identical trees in a model you can select them all, use this extension to shrink the selection to about half, and replace those with another differently looking tree component.
Soure code available at GitHub.
Activated from Extensions > Random Selection.