Topping is removing the main growing shoot, above the desired node, in order to stimulate growth in branches directly beneath where you topped. This can be done to attain 2 or 4 main colas, or to create a bushy plant.
I prefer LST (tying down) and supercropping (controlled bending/snapping) to fimming or topping because I don't like removing the largest budding portion of the plant (the apical tip of the meristem, what you are removing when you fim or top). There are many ways to manipulate the plant into growing however you want without actually cutting away anything. Having said that, I do remove the lower branches that receive little light in order to increase bud production up top where the hps is abundant.