To achieve extraction in fermentation requires contact between the fermenting juice and the skins that tend to float to the top. The more contact and mixing up, the more extraction there is (think about a teabag in a cup of water). In small lot winemaking, especially with thick-skinned varietals as Cabernet Sauvignon, it can be difficult to get enough mechanical mixing to maximize extraction. Color or tannin extraction enzymes can be added during fermentation to help break down skins for more extraction without being too aggressive with the fermenting must (note that this is an especially good idea if the seeds are still a little green and tannic).