Timed World Modifiers pick some random nodes from time to time and check if a TWM was registered for that kind of node and execute it if the chance test passes. Node at the surface or ceiling are a bit more likely to be selected than deep ones.
TWM aims to avoid registering a dozen of ABM that could trigger on a wide range of nodes (for example ground nodes) for non-critical effects. It picks a node, assuming it is likely to have a TWM assigned to it.
This mod doesn't add anything by itself. Check out other mods using the library for effective changes.