Each vine has its own story and all elements around it have to equal it's future quality of wine - it's simply a question of balance. It needs just the right amount of leaves to ripen it's grapes - if there are too many grapes they may not ripen - it needs the right amount of moisture - and in Europe we deal a lot with dry farming so this may also be a measure the farmer has to take into consideration. Should it be a cool year, thinning out the grapes is also a measure to ensure more aeration around the clusters, so thinning tends to be more severe in cooler years.
When it comes to how to prune, it's a selection of the thinnest. The clusters that are the furthest away from the trunk usually get sacrificed as they have less chance of ripening.
In the olden days in Tuscany this was not an operation that was known to farmers. In the past the only value which was appreciated was the quantity of wine made. Crop-thinning was a practice started in Bordeaux in the mid 70'ies and has been adopted in all quality wine region around the World.
Today, quality is the main focus of our DOCG regions in Italy and the operation is today done whenever necessary - and always in July/August before the grapes mature completely as not to waste too much of the plants' energy.
The cut grapes are sour and unripe and can't be used for much so they are left on the ground to compost.
(these pictures were taken yesterday during a wine tour in the Chianti Classico region - 2014 has been a cool year so far so the pruning is quite drastic in some vineyards).