Definition - What does Punch-Down mean?
A punch-down is the process by which the layer of skins, seeds and stems that float above the juice or wine is broken up by submerging them into the liquid during fermentation. It imparts color, aroma, flavor and tannin into the wine which are extracted from the solids. This can be done manually or with a pump.
WineFrog explains Punch-Down
It is important to punch-down the skins, stems and seeds (called the "cap") throughout fermentation on a regular basis. Sometimes it is done up to four times a day. It not only helps to impart polyphenols into the wine, but it also keeps an even fermentation and prevents any off odors or taints which might occur otherwise.
The punch-downs can be done using a pump or with a special tool used manually to "punch-down" the solids in the fermenting wine. The preferred method of punch-downs sometimes depends on the quantity of wine or the varietal which is being fermented. For example, in the case of Pinot Noir, it is often better to do it manually as the skins are more delicate and it is often a wine made in smaller batches.