There is no such thing as a "Dyna Action". There is a DynaActionForm which allows you to specify the properties of an ActionForm in the struts-config.xml file, rather than writing a class that extends ActionForm.
An abstract Action that dispatches to a public method that is named by the request parameter whose name is specified by the parameter property of the corresponding ActionMapping. This Action is useful for developers who prefer to combine many similar actions into a single Action class, in order to simplify their application design.
For example, you might have four buttons on the same form. If your Action extends DispatchAction, you can have four different methods in the same action the perform the four different functions without having to write four separate Action classes.