Why and How do you use textile medium with Fabric Paint to paint on t shirts with paintbrush?

Liquitex makes a fabric medium that is supposed to keep the t shirts from getting stiff. While you can paint right on the shirt with a brush and paint,it works better to put a piece of card board underneath and between the front and back of the shirt to keep the fabric stretched so it is easier to make your art. What happens is that unless you use the fabric medium your artwork will get stiff every time you wash and dry the shirt. Go to the liquitex site and download the fabric medium instructions. they are more extensive than what comes with the bottle.