Punching down is the term used to keep the fermenting wine and all the other solids (like grape skins, pulp, stems and pips) mixed up during red wine fermentation. As the wine ferments/macerates, the sugars in the juice are converted to alcohol and CO2 is formed as a by-product. The Carbon Dioxide bubbles provides buoyancy to all the solids, which rise to the top of the tank and form a thick "cap.