A plant's response to a cut or wound is not unlike that of a human.  The plant bleeds a sap or resin from the wound which dries to form a protective shield.  the healing process continues long after the cut has been sealed and consumes considerable amounts of energy.  Naturally, whilst energy is being diverted to heal wounds, it is no longer available to the plant to fuel normal growth.  

"The importance of good clean pruning cuts can, therefore, never be over-stressed"

Before you start pruning trees and plants, learn how they grow. Trees and shrubs put on new growth each year from the ends of the branches (terminal buds) and from side branches (lateral buds). A plant’s direction and rate of growth are determined by its terminal buds. Lateral buds form branches and twigs that fill in the skeleton of major branches. Dormant buds, which are much less obvious and sometimes hidden below the bark, held in reserve. They only begin to grow if the plant suffers injury to its terminal and lateral buds. A key to skilful pruning is learning how to take advantage of lateral and dormant buds in redirecting growth or rejuvenating a plant.