Pollarding 
This is the removal of the top of a tree at a certain height. It can be used on young trees to encourage multi stem branching from the point where it is cut. If your tree has already been pollarded, it is good practice to re-pollard it at regular intervals. Depending on the species, re-pollarding should take place every 3-5yrs. If this does not happen, then the union between the new growth become structurally unsafe.