Most of a hookah's flavour depends on heat. Too little, and the smoke is thin and tasteless; too much, and the tobacco burns, giving a harsh, bitter taste. Keeping that balance is the master's main job.
Natural coconut-shell coals burn longer and cleaner than quick-light coals, which contain additives that can affect the taste. Coals are lit fully before use and moved, added or removed as the session goes on.
Many lounges now use heat management devices: metal caps that sit on the bowl instead of foil. They spread the heat more evenly and make it easier to adjust, so the flavour lasts longer.