How does Lumped Port work ?

Hi
I have a question.

A wave port sends out energy normal to the suface and it is confined in a waveguide. so I could visualise s11 to be the power reflected and reaching the port through the waveguide.

But in the case of Lumped port, there are two faces to it, how does it excite and what does reflected power mean ?

Thanks in advance.

Guna