There’s no widely recognized photography training method by that “glass of water” name in common camera instruction. It may be a story or borrowed from other balance-training ideas, but the community answers don’t identify it as a standard photographic exercise.

What is commonly taught instead:

• Hold the camera with good support: keep your arms tucked against your body or brace against a wall, tree, or other solid object.
• Use smooth shutter technique: press the shutter gently to avoid jerking the camera.
• Control breathing: similar to marksmanship, steady breathing can reduce movement.
• Use an appropriate shutter speed: camera shake becomes more visible at slower speeds; a classic guideline is to use at least about 1 / focal length on full frame.

One practical drill mentioned is a mirror-and-light exercise: mount a small mirror in front of the lens, point the camera so a flashlight beam reflects onto a wall, then practice your shooting stance and shutter press while watching whether the light spot moves. The goal is to minimize movement during the press.

So: the “water glass” idea is not a known standard photography technique, but there are real, useful ways to train steadier hand-held shooting.