One of the upcoming new feature in darktable is the ability to use the same development module several times. By applying the same module multiple times and combining them with blendif it is possible to do some effects that could not be achieved previously without using external tools like the gimp.
Modules that can be instantiated multiple times have a new icon in their header, next to the “reset” button. Clicking that icon will open a pop-up menu that allows you to create a new instance of the module, change the order in which the different instances are applied, or delete an instance of the module.
This is a companion discussion topic for the original entry at https://www.darktable.org/2013/02/multi-instances/