Originally Posted by lab fairy
Sounds like a version of a tailor's seam clapper. Wooden pressing sticks are also useful for narrow seams. Both of these items have been around the garment sewing industry for a long time.
The intent of the clapper/mallet/hand-twist is to loosen the thread weave at a seam. As noted in several of the replies, gentle steam helps. Of course, one should never pound hard enough to damage the fabric threads or the seam thread.
For the seriously cheap, take a 5-6 inch piece of 2x4, sand it well, and use the narrow side for your clapper. Handle is optional, although recommended if you do this a lot.