Once the decay starts in a stub-cut branch, it can spread into the trunk, weakening the structural integrity of the tree and ultimately leading to its death. The process is insidious because it's slow. In most cases, the proper pruning cut is a thinning cut. A thinning cut removes a lateral branch back to its point of origin. Proper thinning leaves no stubs.
Arborists learn more about trees all the time.
We used to tell you to make pruning cuts flush with the trunk. Cuts made outside the branch collar result in less decay than flush cuts. I am encouraged by the attempts of tree pruners to avoid drastic topping. When we overcome that obstacle, all our trees will be safer. For further information contact your local WSU Cooperative Extension Office .