What is a crown thin? Like a crown reduction it is work carried out on the canopy of the tree. However, that’s where the similarity ends. A crown thin does not reduce the size of the canopy of the tree but merely removes branches from within the canopy. Why would you do this? To allow light through the canopy by removing density of foliage. 

This can be a good route to go down if you have a mature tree for example that is protected with a Tree Preservation Order that dominates your garden. It is unlikely that you would be given permission to remove or considerably reduce it. By thinning the tree you allow light through the canopy and sky to be seen through it creating a much more open feel to your previously overshadowed space.