Punching down is done to keep the fermenting wine and skins mixed up during red wine fermentation. As the wine ferments, the skins rise to the top of the tank and form a thick cap.  Breaking up the cap a few times a day increases the extraction of color and flavor, and prevents the cap from drying out and/or developing bacterial problems. An instrument used to push down the cap that looks like a huge potato masher (or might actually be a huge potato masher) is called a punch or punching tool.