Need a detailed explanation or understanding of respiratory biofeedback devices
If I have understood the question correctly, in the TT equipment for example in the lower part of the chest special straps/belts are placed. They react to the inhale and exhale.
This webpage may be helpful for You:
http://thoughttechnology.com/index.php/respiration-sensor.html
One of the types of biofeedback used for respiration of capnometry. For some people, looking at end title CO2 levels and using that as a feedback signal can be helpful.
For disordered mechanics of respiration, surface electromyographic feedback can be used to train accessory muscles etc that are used in disordered respiration.
Respiration belts used on the chest and abdomen are also very commonly used.
There are many diseases and apps that are sometimes called biofeedback that don't take any physiological measurements at all, and instead work by instructing the user how and when to breathing. Though most would not consider these to be biofeedback, they are sometimes used for the same purposes.