Well, here is an article from 2002 explaining the idea: Forestry Operations There is nothing new under the sun That was pre-C3C, where a forest chop still took 10 turns. Now in C3C, where forest chopping costs only 4 turns, this technique is of course much more powerful. You can harvest each tile only once. So if there had already been a chop on a certain tile, planting a forest and chopping it again, will give you nothing. So in the beginning, people had to draw little maps while playing, keeping track of which tiles had already been harvested and which not... So tedious. Nowadays we fortunately have CivAsstistII, which provide this information.