Why can the ng-sound, as at the end of the word sing, be beneficial to your vocal development? On the ng you can vocalize up and down, do scales and slides. The tongue is high touching the palate. With it you can safely sing both low and high because you really can´t force the voice while “ng-ing”. It gives a nice stretch to the vocal folds and helps register shifts. Also, it trains the awareness of higher voice placement, the head voice, the feeling of sound high in the head but without pushing too forward.
How should the ng be executed? Let the tongue rest quite forward, away from the throat. Open the jaw while ascending the scale – it is possible to vary the degree of mouth opening according to pitch while doing ng. Remember all the good things about posture and breathing and body connection. Then just let the voice glide easily from note to note without h´s in between.
When you learn to “ride” on the ng-sound you can even imitate the movements of articulation while doing it. That way you gain awareness of the sound staying up high up in the pharynx even though you pronounce words. It´s a bit funny, of course, but can be a useful training trick for classical singers. Pop singers want to have the voice sound more like speech but even they can benefit of the flexibility the ng can give.
Sometimes singers fear that the ng will raise the larynx too high. That can be the case if you cannot loosen the space between the hyoid bone (tongue bone) and the larynx. That space helps singing so a stretch in that area should be possible. So, don´t swallow the tongue while inhaling, rather bring it gently up away from the throat. If your inhalation is deep it will also relax the larynx and help it descend.