The porous jump is a special scenario where the force transferred does not include any viscous forces. The pressure drop obtained from the parameters such as thickness and coefficient defined at the porous jump condition is converted to a force vector, which is transferred via system coupling. 

If you wish to read further on this, I suggest you use the following link from the Fluent User's Guide (section 42.4.2. Force transferred to System Coupling from a Porous Jump Boundary)

42.4. Variables Available for System Coupling (