The main concept behind this is believed to be their assistance in keeping an airway straight and open. There is also some thought that it helps keep chin/mouth closed and there may be truth in both of these.
At one point I was diagnosed with apnea via a home sleep study but since then I have had two PSG's claiming I do not have either apnea or UARS (UARS would be the only possibility but imo isn't the issue) although I do have poor sleep quality (spontaneous arousals). I continue to use my CPAP machine off label because I do find that I sleep better with it and I believe the humidity it provides is a reason for that. Anyways I am including this paragraph as a statement that I do not have known apnea let alone positional apnea and supposedly have near normal sleep breathing.
2.5 weeks ago I was involved in a work accident in which I fractured my c6 neck vertebrae in two different ways and since then I have been wearing an Aspen Vista hard cervical collar and will continue to do so for a minimum of 6 weeks. This collar holds my neck straight and it also holds my head in position relative to my neck.
Interestingly I have noticed a minor change in my OSCAR data and thought it worth sharing.
Here is a view of 6 nights of flow rate data before my accident. There are some user flags and the odd apnea (most post arousal but the very odd real one) but you can see some areas that it looks like flow rate was declining and potentially ending up in a RERA as well as a couple periods of what appears to be high expiration effort. Nothing crazy just the flow data from someone with mediocre sleep quality tossing and turning and hitting different body and neck positions while sleeping.
Here is a view of 6 nights with the collar. My apnea and flow limitations is perhaps down very slightly (not statistically relevant) and the user flags is down somewhat noticeably but the main thing that caught my eye was how consistent the breathing is and how there are next to no obvious periods of restrictions, RERAs or high exhalation effort etc.
This agrees with some of my previous sleep video recordings in which I noticed that many times when I have an obvious restriction/RERA it is because I have slid down on pillow and my neck is kinked into a strange position.
Anyways I just figured some people might be interested in seeing that people even with near normal sleep breathing can see a difference in their breathing by controlling airway positioning using a cervical collar or other methods (pillows etc) to keep neck and airway inline and open. I definitely wouldn't recommend everyone try a hard collar out as they are far from comfortable but I guess it could be tried in cases where people have obvious positional apnea and can't seem to get a soft collar or anything else to work. If I had to wear a hard cervical collar in order to get good quality sleep I am pretty sure I could figure out a way to get used to it.