In a refrigerator held at, say, 3 °C, the evaporator may run a few degrees below freezing and collect light frost. When the compressor stops, either at the end of a normal cycle or during a scheduled defrost, the evaporator fans keep moving cabinet air across the coil. Because that air is above 0 °C, it melts the frost and the water runs into the drain tray.
Off-cycle defrost is simple and efficient, because it uses no heater and the meltwater partly re-humidifies the cabinet, which is gentle on fresh produce and cakes. It only works where cabinet air is warm enough, so it is used in chillers, display cases and beverage coolers, not in freezers.
If a chiller still ices up, typical causes are door gaskets letting in humid air, a setpoint set too low, very frequent door openings in humid weather, or a controller configured with too short an off time. A technician can lengthen the off period or adjust the defrost interval.