The apical bud (or tip) produces the growth hormone auxin, which not only promotes cell division, but also diffuses downwards and inhibits the development of lateral bud growth which would otherwise compete with the apical tip for light and nutrients. Removing the apical tip and its suppressive hormone, allows the lower dormant lateral buds to develop, and the buds between the leaf stalk and stem produce new shoots which compete to become the lead growth. Manipulating this natural response to damage (known as the principle of apical dominance) by processes such as pruning (as well as coppicing and pollarding) allows the horticulturist to determine the shape, size and productivity of many fruiting trees and bushes.
Some fruit trees have strong apical dominance, and young trees can become "leggy," with poor side limb development. One can reduce the apical dominance in this case, or in cases where limbs are broken off by accident, but cutting off the auxin flow above side buds that one wishes to stimulate. This is often done by orchardists for young trees. Select the bud along the leader (stem) where one desires a side branch to develop, or where one already is present, but growing too weakly. With a sharp knife make a horizontal cut about a half inch above it, just deep enough to break the cambium layer, and only about a quarter of the way around the stem. This breaks the flow of auxins that had suppressed its growth. Later, when a bud breaks, it can be trained or pruned as needed.
See also Pruning fruit trees