From what I understand, the clapper is used to absorb the moisture of the the steam whilst pressing down on the seam. The drying out of the fabric whilst holding the seam flat gives the required result. The 'clap' comes from the noise of quickly getting it onto the seam as you remove the iron. It is not used as a hammer by repeatedly clapping on the seam.
I use my rolling pin method with a dry iron, but I guess it doesn't give the professional finish given when using the clapper. It is just better than using a flat ironing surface.