The D-Cup Diffuser is a miniature hydro-cyclone which has been specifically designed and 'tuned' for the removal of air from the seed distribution line just above the point of sowing.
The best way to visualize the working of the diffuser is to imagine the air being 'spun' out of the centre of the top, while the seed / fertilizer travels down the outer walls to the outlet.
Click Here for a detailed overview of how it works.