Instead of using heaters, a hot-gas system opens a valve that sends hot discharge gas from the compressor straight into the evaporator. The coil warms from the inside, frost melts quickly and evenly, and the heat comes partly from the compressor's own work rather than from separate elements.
Hot-gas defrost is faster than electric defrost and can use less energy overall. It is common in larger remote systems, cold rooms, ice machines and some high-end cabinets. It needs extra valves, piping and control logic, and the system must be designed so that liquid formed during defrost does not return to the compressor and damage it.
For small plug-in cabinets, the added complexity is often not worth it, which is why most use off-cycle or electric defrost. Where hot-gas defrost is used, valve faults can mean incomplete defrost or a coil that warms slowly, so service by technicians who know the specific system is important.