A monitor that halts when a metric crosses the high bound — and stays halted until it drops below a lower one. That gap is hysteresis: a signal hovering at the edge can never flap the halt on and off. Slide the metric (or tap) and watch the little machine hold through the chatter.
A hysteresis (Schmitt-trigger) stopping rule. State is ARMED until the metric exceeds HIGH=0.80, then TRIPPED until it falls below LOW=0.55 — and only then ARMED again. Because the rearm bound sits below the trip bound, a value oscillating around 0.80 stays TRIPPED, never chattering. The state carries across frames, so sliding up through HIGH and back to the 0.55–0.80 band keeps it tripped — the asymmetry you can feel. A fail-loud self-check throws unless a fixed sequence trips above HIGH, holds through an edge-hover, and rearms only below LOW.
One metric, two fixed thresholds — real monitors watch many signals with tuned bounds and debounce timers. The bistable transition rule is exact.