Well, I took the plunge. From the thesis:

The diffuser introduces the pressurant gas into the propellant tank as a critical component of the pressurization system in a liquid propulsion engine. Element present in both pressurization systems (self-pressurization and by inert gases), the diffuser makes the pressurant enter the propellant tank at a desired direction and velocity to keep the pressure inside the tank at the design level during the pressurization activities without the engine working (on-ground operations or during coasting phase) and to avoid the creation of zones where the operating pressure falls below a threshold value of NPSH during engine firing.