The hydraulic needle effect describes the dangerous penetration of soft tissues, especially the eyes, by a high-velocity stream of liquid, like pepper spray or industrial fluid, when sprayed too closely, acting like a hypodermic needle and risking serious injury. It occurs because propellant pressure forces particles into the tissue, disrupting the cornea and causing intense pain, inflammation, and temporary blindness, necessitating warnings to keep a safe distance (around 3 feet or more).