This is a part of the body above the sexual organs, that Chinese thought says is the focus of energy. It is located very close to the source of kundalini energy (Indian thought).
If you’re able to tuck your tan tien proper, your posture will straighten immensely. My initial perception is that this can increase highs because proper posture alignment helps to send the sound up through the sinus cavities.
Tucking in the lower abdomen will also solve the lower backache problems (because less continual tension on lower back muscles and ligaments–continual tension is usually the source of lower backaches), prostatitis pain (as explained, much of the pain of prostatitis is from the muscles surrounding the prostate), sunken chest, acid reflux, excessive saliva while singing).