All topiary trees need to be pruned annually because the tree would otherwise revert to its natural shape. An example of this is the espalier linden, which is originally a tree with a regularly branched crown. Sometimes it is possible that at a particular level no individual branch is satisfactory for training, and thus creating the right final picture. In that case, make a small nick in the wood with a knife. This interrupts the flow of sap, and the nutrition accumulates. This influences the formation of buds (and branches), allowing you to hide that blank wall from view.
Download the pruning guide