Merrill Tenney gives his observation about the work of a vinedresser and the pruning process in his commentary on John: 

In pruning a vine, two principles are generally observed: first, all dead wood must be ruthlessly removed; and second, the live wood must be cut back drastically. Dead wood harbors insects and disease and may cause the vine to rot, to say nothing of being unproductive and unsightly. Live wood must be trimmed back in order to prevent such heavy growth that the life of the vine goes into the wood rather than into fruit. The vineyards in the early spring look like a collection of barren, bleeding stumps; but in the fall they are filled with luxuriant purple grapes. As the farmer wields the pruning knife on his vines, so God cuts dead wood out from among His saints, and often cuts back the living wood so far that His method seems cruel. Nevertheless, from those who have suffered the most there often comes the greatest fruitfulness.