Oh, good gracious, I believe every piecer could benefit from the use of this tool. This fabulous tool not only flattens bulky seam intersections but it also relieves stress. Yes, that's an unexpected perk!
Simply press bulky intersections from the right side of the fabric as usual, then steam, steam, WACK, WACK, steam and voila ... a beautifully flat seam intersection. Unsightly bulges are gone and it's easier to quilt this area of the quilt as well. (Doesn't it make you crazy when the machine skips stitches while trying to stitch over bulky seam intersections?)
I used to perform this task with a metal meat mallet but that can crush fibers. A wooden meat mallet turns some fabrics shiny. The inventors of this tool tested to be sure fabrics won't be damaged with the Wacker. (Of course, I recommend testing before wacking non-cotton fabrics.)
This tool is called out in Precision & Panache.