This tool is pretty simple but really works. Basically it's two pieces of plastic cutting board material. The two pieces together weigh a little over 13 ounces and measure 6 x 4 x 1/2 inches each. They're also textured so they have a little "grip" to them. Just put the wire piece you want to flatten "harden" between the two pieces and hit them with your hand or a rubber mallet.
From the manufacturer: "Press & harden your wire designs using the Wire Whacker with your hand or hammer! Easy-to-handle, simple-to-use will work with most wire gauges and the wire diameter will not change. Won't mark or nick most types of wire, including: Gold, Gold-Filled, Sterling Silver & Copper. After placing your wire designs between the 2 Wire Whacker boards, whack the top board with your hand or hammer. Your wire designs will harden and retain their shape!"