Originally Posted by Tomguy

The flush machines are needed for vehicles like my 300M - no torque converter drain plug, but you can work around it. For example, drain the pan and replace the filter, put in fresh fluid, cut the line to the trans cooler and put the outlet in a jar. Put the inlet in a fresh can of ATF. Start the car & run until clear fluid comes out the draining end, then splice or replace the line.

The machines typically just take the place of starting the engine in the process, and use pressure. It can damage seals in the trans, because the inlet is usually pulling versus being pushed.