and when I say 'cardboard', I simply mean a stiffer grade of paper or card as thin paper will simply bend and flutter from the paint and air pressure from your airbrush.
I think it is not for painting with a brush.The technique is for airbrushing.The secret of it is that you have to follow the masked edges holding an airbrush sheer to a model surface.
But for brushig it is possible to achieve a similar effect but the way is different.