Once fermentation is underway, we punch down the cap twice a day. Our little friends, Saccharomyces cerevisiae, are chomping away on grape sugars and turning it into ethanol, CO2 and heat. The CO2 that is released pushes the grape skins to the surface and the heat "bakes" the skins into a soft cake that we call the cap. Above you can see that half the tank is punched (look at all that creamy goodness bubbling up - thats CO2 coming up) and the other half is still caked.
The problem with a cap is that the skins are where all the flavor, color and tannins are so we need to push the skins back under the juice occasionally. It also has the effect of helping the fermentation lose some excess heat which it especially important in these wooden tanks since the staves hold their temperature well. At Kosta Browne we punch down twice a day during fermentation.
I love doing punch downs. The smells and the exercise remind you that wine is a living thing and keeps you close to the wild side of nature. Because at the end of the day, wine is just rotten grapes!