Auxin is a plant hormone produced in the stem tip that promotes cell elongation. Auxin moves to the darker side of the plant, causing the cells there to grow larger than corresponding cells on the lighter side of the plant.

These new sideways shoots also lay down their own buds which in turn, are kept dormant until the shoots’ own growing tip has extended to a predetermined length (determined by species). In bonsai you force the tree to produce this natural reaction; by cutting off the tip you stop the auxins in their track thus making a new shoot develop behind your cut.