The Hot Flush machine pumps heated cleaning fluid through the oil cooler, with the direction of flow instantly reversing every 3 seconds for a quarter of a second.
The machine is ran until the 10 Micron check screen on the out-bound flow shows no debris. Hot Flush does not consider an oil cooler clean until nothing larger than 10 Microns in size is coming out of it. Compare this with the filtering efficiency of your average spin-on type, disposable oil filter which is effective down to a particle size of roughly 30 Microns.
Most oil cooler manufacturers have a recommended overhaul interval of every 300 to 400 hours of operation, or every two to three years (whichever comes first). Engine manufacturers generally recommend that the oil cooler be overhauled along with the engine at the end of each TBO (time between overhaul) cycle.
Hot Flush should be used anytime there has been metal contamination of the engine oil, whenever any major engine work has been conducted, or in the event of abnormally high oil temperatures when no other causes can be found.