Transformations can be applied on demand in your model as a sort of design pattern. You can design a pattern using the transformation feature and "play" it in your model in order to modify objects.
For example, if you are working in an OOM, you can create a transformation that converts all analysis classes with the <<control>> stereotype into components in order to add an implementation layer to your model.
You can apply transformations through a command added to a main or contextual menu (see Customizing and Extending PowerDesigner > Extension Files > Menus (Profile)) or by opening the Apply Transformations window from the Tools menu.