It is quite simple however it is also difficult to implement. You also need to consider what is "Full Range". Very very few speakers reach into sub bass territory yet most people consider most box speakers full range. Most speakers reach into the 40-50hz flat region and that can be accomplished via rear horn with a good AER, Voxativ, Lowther or even a Lii Audio but I am only a fan of Lowther if rebuilt by one of the gurus or the Field coil version.
You need 2 things to effectively pull it off and it leads to a driver that needs to be implemented in a very specific manner. I am a member of the "Lowther Modification" group on Fb where several of us , myself included make our own high eff drivers completely from scratch so I know these driver types intimately.
I also own AER BD3 drivers in addition to the ones I build and 5 other FR drivers and there is a definite trend in all of them.
In order to reach the very high frequencies in a large driver you need a VERY strong motor and a VERY light cone. The cones in these have employed long fibers, stiffening ribs and rolled edges for quite some time and those features are just now becoming standard in ScanSpeak, Eton and others as they play catchup.
The strong motor, light cone formula leads to a rising frequency response with a VERY low eletrical Q that is best used in a back or front loaded horn. The rear horn will give the best low frequency response from a driver that barely needs to move to make bass. As the motor strength decreases so does the HF extension.
If they are not in a rear horn then bass response will suffer along with efficiency.
The really good drivers are not cheap and should not be confused with the cheap kit drivers. The real good drivers start at $10,000 a pair and that is just the driver.
While back loaded horns and "full range" speakers of this type are not as popular in the US there is a new US manufacturer, Songer Audio, that makes an outstanding field coil "full range" speaker w open baffle and integrated sub that I heard 3 weeks ago in the LA show that is incredible.