hfss waveguide port excitation

how do we excite a waveguide in HFSS. The excitation is given by excitation->waveport->subsequent windows.
Actually what is happening exactly to excite the waveguide in HFSS apart from the general mentioned way of excitation as above.

Source: HFSS Help Menu

Wave ports are typically placed on a interface to provide a window that couples the model device to the external world.
HFSS assumes that each wave port you define is connected to a semi-infinitely long waveguide that has the same cross-section and material properties as the port. When solving for the S-parameters, HFSS assumes that the structure is excited by the natural field patterns (modes) associated with these cross-sections.