Bonsai defoliation involves cutting all the leaves of a tree during the summer. In doing so you force the tree to grow new leaves, leading to a reduction in the size of leaves and an increase in ramification. This technique should only be used on deciduous trees healthy enough to withstand this demanding technique. Defoliation can also be used to reduce growth in specific parts of the tree; you might for example defoliate the top part of the tree while leaving the bottom part untouched, in doing so you restore balance in the tree.
For most bonsai tree-species June is the best month to defoliate, leaving enough time to grow new leaves and prepare for the winter season. The exact moment depends on the tree; defoliate after the new spring-growth has hardened off.
Most deciduous trees can be defoliated, but be sure to check this as some trees will not react well to this technique. Do not defoliate weakened trees (due to recent repotting, pruning or disease) or trees that are still in training.
Cut the leaves using a twig shears, leaving the leaf-stalk intact (see photos 1-3, below). Using the right Bonsai tools will help significantly.
A defoliated tree does not need specific aftercare. When only partially defoliating a tree (for example, only pruning the top part of the tree) you better place the tree in the shadow for about a month to protect the exposed interior leaves. Also, in areas with very strong sun you can shade your defoliated trees to protect the bark from getting sunburned.