My reply at HCP to a member with regards to why/how the device works.

Wala O2 sensor ang older cars.

Usually they will be taking idle fuel delivery based on TPS, MAP/MAF, ECT & IAT & reference it to a preprogrammed map which is programmed to ensure sustainable combustion under the worst conditions(hardly ever tuned up engine in cold weather with a clogged air filter for example) that would normally mean the idle mix is richer than neccessary.
Which is also why it's very effective on vehicle with "primitive" fuel metering systems.
That being the case a bypass like that would save fuel at idling but as mentioned at the expense of increased NOx levels.

Someone mentioned that there was an adjustment knob.
That would be neccessary to make the unit universal.
Let it too much air thru the device & it's like having a vacuum leak in your intake system.
We all know a big enough vacuum leak is detrimental to your engine's operation.

Device's like that work only if you spend a lot of time idling, like when your stuck in traffic.....
Are we getting ideas yet? ;)

Try using that on a vehicle equipped with 02 sensors & on the open road & see what happens.

I know I won't be getting any miracle fuel saving device anytime soon.
I've been around too long. :P