When grapes used to make wine are ready to be harvested, the winemaker can choose to pick by machine or to pick by hand for many different reasons.
Hand-picking is nearly always preferred for wine grapes, or for smaller vineyards that can't risk losing any fruit. For handpicking, all you need is a pair of secateurs and grape baskets (small bins) spread throughout the rows underneath the vines.
Quite simply, you clip or cut the bunches off each vine and place them into the baskets, making sure not to damage the vines and most importantly the grapes on the bunch. Then you carry, transport the grape baskets down the rows to possibly larger containers, or carefully packed onto a truck to be transported back to the winery.
Despite the added costs with hand harvesting, many wineries prefer the use of human workers to hand-pick grapes. The main advantage is the knowledge, skill and discernment of the worker to pick only healthy, ripe bunches throughout the vines and the gentler handling of the grapes.
The production of some dessert wines like Sauternes and Trockenbeerenauslese require that individual berries are picked from the botrytized bunches which can only be done by hand, as they will split during machine harvesting; this also includes 'late harvest' and 'ice wines'.
Hand Harvesting allows individual bunch selection based on ripeness - therefore fruit selection can be more exact, often selection is by single berry rather than a bunch and can be done repeatedly over many days, even weeks.
Hand picking can be employed on any terrain although unfavourable terrain (steep slopes, terraced hillsides) dramatically slow the progress and therefore increase the cost of doing this. Hand picking is also needed when vines are grown in a Pergola style - like those found in areas of northern Italy - also where you have very old, fragile, low cropping 'bush vines' that grow at different heights and in irregular patterns.
A good picker can harvest up to 2 tonnes a day. Hand picking is becoming quite common in New Zealand for premium wines where a great deal of time and energy has been invested in getting small, high quality fruit from carefully selected sites and rows. Plus some regions in Europe have regulations that do not allow mechanical harvesting (e.g. Champagne).