Running out of air before the end of a phrase? Voice shaky when you sing quietly? These are signs of inconsistent airflow.
The sustained hiss strips away pitch and melody so you can focus entirely on the engine of your voice: breath support. This is the simplest test of whether your support system is working.
The Sound
Sharp, consistent "Sss" like a snake or leaking tire. Loud enough to hear but steady. No wavering or pulsing.
The Feel
Hands on your lower ribs (the floating ribs). Breathe in and feel them expand into your hands. As you hiss, keep those ribs expanded as long as possible. Your abs move slowly inward to push air out, but your chest and ribs resist collapsing.
The Drill
Sustain the hiss for the full duration. The goal isn't how long you can go, it's how evenly you release the air.
Why This Works
This isolates your expiratory muscles (abs and internal intercostals) from the larynx. If you just let go, air rushes out instantly. The sustained hiss teaches your body to create resistance against that collapse. That "braking action" is what singers call support.