Maybe I can clear up a few things.
Automatic transmissions must have a pump in them to generate hydraulic pressure to make the transmission function. This pump will both suck fluid and push fluid.
When you use the hose in a bucket method to flush the trans fluid, you disconnect the upper trans cooler line from the radiator and add an extension and drop it into a bucket. When you start the engine, the pump is functioning and will pump fluid from the trans into the bucket. You don't want to damage anything in the trans from lack of lubrication, so you must also add fluid as it is pumping out. I prefer to add fluid via the dipstick tube. Another similar method some have used is to simply connect another hose to the cooler where the first hose was disconnected and drop this second hose into another bucket with fresh trans fluid. The pump will suck new fluid out of this supply bucket as it pumps old fluid into the waste bucket. I don't care for this method as I feel that it is harder to control, but it does work and some prefer it.
It is a very rare thing to flush a trans and cause problems from flushing it. There just about had to have been a problem first. It is unwise to do a trans flush without first road testing the operation of the trans. I have flushed many with fluid that was darker than old motor oil and not had any problem from it.
Anyway guys, keep arguing if you wish. This is just my observations based on my own experience.
