I was fitted for one, and was told that the lower jaw is held forward by using the upper teeth as an anchor. Over time they can cause upper and lower teeth to move.
For those who want to simulate the feeling, touch your tongue to the front of your top teeth and jut your jaw forward so you can close your jaw with your tongue staying in place, sandwiched between the front of your top teeth and back of your bottom teeth.
It was wildly uncomfortable and I gave up after three nights. Also didn't notice better sleep.
Wow, I can (just barely) slide my bottom teeth in front of my top teeth, but not with the tongue in between, and it still feels extremely weird and uncomfortable...
To see a photo, image search "apnea sleep guard." By jutting your lower jaw forward, it is supposed to move your whole lower jaw assembly including tongue forward. It is the tongue drooping back towards your throat that causes the snoring for most. This, in turn, should reduce episodes of reduced oxygen then leading to less wakeful events since your body is not having to jerk itself awake to keep air flowing.
I have one, and it works well. It's the default treatment here, where CPAP is reserved for the morbidly obese.
But the effect of jaw displacement is real. Every morning it takes a few hours to move the lower jaw slowly back. And I've visited a dentist in another country, who advised against it (and had apparently rarely seen it).
It was wildly uncomfortable and I gave up after three nights. Also didn't notice better sleep.