Any sort of ordinary deep frying. A volume of oil, deep enough for deep frying, is floated on a volume of water in a temperature controlled vessel. In the more advanced cookers, the oil, water, and oil/water boundary are kept at separate, "ideal," temperatures through a rather complicated and Rube Goldberg type of arrangement. In the less expensive, less complicated cookers the water and boundary temperatures are either not actively controlled at all, or not monitored as closely.
The idea is that solids sink to the bottom, through the boundary, are trapped in the water, and do not continue to cook in or pollute the oil. The principle imputed benefits are less oil lasts longer. AFAIK, there are no claimed health benefits, but only the economics and "green" of saving oil, and the implied quality benefit that an operator won't ruin food by using old oil.
My impression is that at the current technological SOTA, water fryers are either expensive, unreliable, or more usually both.
Hope this helps,