people come back to the explanation because they understand how a traditional sail boat works, and many other people don't. when discussing the issue detractors inevitably invoke the workings of traditional sail boats, which naturally makes this description enlightening. of course it is not required from a pure dynamics point of view, where you simply need to describe the end result of your propulsion system. it just so happens that the airfoil is the primary way REAL devices interact with gases, so one endeavors to understand how the specific case works, not just the general, free body diagram...

I am an HVAC designer by trade, and I think that a hot isle containment setup in general is a good idea. especially if you are trying to use something like curtains as your containment. when you have a standard CRAC unit, they have been until recently only constant airflow volume units. but the servers themselves have their own fans that stage or change speeds to move the required air through the racks. unless the airflow from the CRAC is being varied based on demand, the extra airflow will just spill out past your curtains, out the ends of the isles and cool the room outside the cold isles no matter what you do. newer approaches (including the Liebert approach) advocate sensors on the racks to calibrate demand, and do match airflow to the load. In these cases, containment is not strictly necessary, because the racks are transferring nearly the same air through from hot to cold as the unit is supplying and returning. the natural air convection serves to separate streams pretty well. but, you could gain some efficiency on the CRAC unit cooling side by containing the hot isle air, the idea being to bring back the warmest air possible. obviously the air mixing with cold air is a source of wasted energy, but also, the refrigerant cycle can operate at a warmer relative temperature for a given amount of cooling. when you require the cooling coil to operate colder, the cycle efficiency drops off. Of course, there is also energy saved by not operating the fan itself at such high volume. but, for my money, variable air volume and proper controls to match airflow at the unit is worth more than trying to set up containment, especially since the curtains or other devices make for a less flexible layout, while relocating sensors to critical racks (the ones that get hot first) can be done quicker.