This is usually done to old non productive fruit trees. There are two ways of doing it. 1. Severe root pruning. 2 Ring barking.
So to root prune you take a spade and find some of the main roots coming from the tree and sever them. The first year you do this to one side of the tree and the next you do it to the other side.
Ring barking is less work but needs to be done carefully. You cut a deep groove in the bark of the tree, about half way up the clear part of the trunk, below the first branches, about half the circumference of the trunk. NOT ALL THE WAY ROUND. This restricts the flow of sap etc. up to the crown as does the root pruning and makes the tree think it is dying so it tries to propagate itself by flowering and fruiting.
Both are drastic measures and best done by someone who knows what they are doing.

for walnuts you beat the tree with a stick. it helps the sap rise appaerntly.

I think I will try Ring Barking. Thanks