In the summer time, plants are in full growth and foliage hides bunches, which need light to ripen. It is indeed useful to thin out the leaves in order to make the aeration of bunches and the phytosanitary penetration possible. This is helpful to improve the sanitary state of harvests.
To pull leaves off is also important because it increases the sugar rate in bunches, and thus gives fruity hints and reduces plant flavours.
Workers use a small three-wheel cart to be bunch-high. They move it along grape rows and return it when they arrive at the end.
The essential point to take into account when you thin out leaves is to be sitting in front of the sun when sun is highest in the sky. The goal is to pull off only the leaves, which are in the shade side because leaves, which are in the sun side protect bunches against the burn sunbeams.