You can see the paint is pulled to the top of the creases in the fabric. The fabric on the bottom against the plastic is the lightest, staying damp the longest. Many times I turn the fabric over when the top feels dry, to further dry the underside, but also to encourage a little more creased staining on that side.
This method works great for more contrasty stained fabric with very light overall areas and darker creased stains. For a more overall colorful fabric (like this purple one) brush straight paint or a less diluted solution on damp fabric with a paintbrush, then crumple fabric to dry.
Once it's dry it takes some serious ironing to get all the wrinkles out, usually requiring extra steam or spritzing with water while ironing.
Click here for a photo tutorial of the process.