A Diffusion pump works on the principle of gas diffusion. This pump requires Diffusion Pump Oil, which evaporates, gets compressed and then escapes through jets, trapping air on its path down to the bottom of Diffusion Pump from there the air is evacuated by a Rotary Pump or a Booster Pump- Rotary Pump combination.
For this arrangement to work, vacuum of minimum 10-1Torr in required to be maintained in the main body of Diffusion Pump. Otherwise the Diffusion Pump Oil escaping from jets in gaseous form, collides with too much air available and further raises pressure in Diffusion Pump body. This stops creation of vacuum and is called Stalling of Diffusion Pump.