How does the interection between the Dynpro and the ABAP/4 Modules takes place? | ABAP Module Programming

-A transaction is a collection os screens and ABAP/4 routines, controlled and executed by a Dialog processor. The Dialog processor processes screen after the screen, thereby triggering the appropriate ABAP/4 processing of each screen .For each screen,the system executes the flow logic that contains the corresponding ABAP/4 processing.The controls passes from screen flow logic to ABAP/4 code and back.