Tree pollarding is an old pruning technique, which removes all crown growth of a tree back to the main lateral branches, sometimes even as far as to their axial growing point close to the main stem.
Only certain trees are suitable for this radical "topping" procedure, e.g. Willow, Elm, Chestnut or Plain trees and should only be carried out on trees, grown for this purpose and maintained on regular intervals.
Never be tempted to apply this treatment to any mature tree as e.g. Beech, Oak, Cherry etc. as most likely they will not tolerate same and won´t survive for much longer afterwards !