submerged cap is a way of keeping the cap wet and mixed with the fermenting wine to facilitate extraction. A perforated screen is positioned just under the surface of the liquid to keep the cap submerged but the pressure of the rising carbon dioxide tends to force the cap against the screen, making extraction less efficient. A pipe to allow the gas to escape can help to resolve this. It is still quite common to see head boards performing the same function in traditional concrete tanks in the barossa valley. See also punching down, pumping over, and maceration.
