That is a very confirmed technique to damage the oil pump in the transmission. If you wish to flush out all the old oil, you replace (clean) the filter in the sump and pack it back, fill with the exact amount of oil you drained, remove the out side from the cooler and dump it in a white coloured pan.
Have a person ready with an oil bottle with about 3 or 4 litres of fluid, Start engine and start to pour the fluid while watching the cooler line output volume and colour - as soon as the fluid starts to run clean stop and plug install the cooler line back. Start and check the fluid, adjust as necessary.
Never ever let the trans front pump run dry, it will damage the pump considerably. The above technique can also be used to change the entire fluid if you do not want to remove the sump.