The mechanism is very simple… if you remove a tool and then use it, there will be a DNU.
(it should open instead window and tell that no tool is installed)
In fact, it's worse: it DNUs even if you still have some tools installed, just because you unregistered one of them.
i.e. With AltBrowser installed, you have four browsers in your image: SystemBrowser, Nautilus, PackageTreeNautilus, and AltBrowser. If you unregister AltBrowser, you have a DNU, even if you have the three others still available.
There is apparently some magic in the world menu to trigger a user-driven choice among existing tools, but I don't know how to trigger that.