Crown thinning is the removal of small branches throughout the trees crown. The aim is to reduce the density primarily to allow in light and air, reduce wind resistance and the weight of the crown.
Overall there is no major change to the trees shape because internal branches are removed. The general rule is that no more than 25% of the leaf area should be thinned, mainly from the outside third of the tree crown. If too many branches are removed from the lower two thirds (resulting in lions-tailing), it could effect the structural integrity and cause branches to break in the long term. The size of branches removed should also be limited to around 3-4cm in diameter.