This is selectively removing branches to reduce the density of the crown. It should be achieved by removing branches from throughout the inner and outer crown, to leave a  balanced overall density, but again to reduce stressing the tree no more than 30% of the overall crown should be removed in a single year.

When to crown thin a large tree:

To allow more light to pass through the crown, reducing the effect of shade through the tree.

To allow the wind to circulate through the crown, therefore reducing the bio-mechanical stresses of wind.

To remove rubbing or crossing branches within the crown, which can leave wounds open to infections and diseases.

When not to crown thin a large tree:

Crown thinning of certain species can have the effect of making them produce more growth along the branches, at best undoing the recent crown thinning or at worst making the crown even denser than before you started. Species such as Limes, Maples and Willows can certainly be a risk.

Where a tree is already unstable due to a defect at its base or due to soil/root movement. A crown reduction may be a more appropriate recommendation in this instance.