Undercuts are used to remove large branches, that if you cut it from the top, the weight would snap it off. This can lead to tears in the bark and expose the tree to diseases. To undercut, start under the branch and saw upwards until you have cut halfway through. Then saw down from the top of the branch, starting about 3-5 cm up from the undercut. Keep sawing until the weight of the branch pulls it away from the tree. Tidy up the tree by cutting from the top of the remaining branch, away from the trunk, all the way through.