I''m not the best one to answer your question, but here goes. Your sail is an airfoil (unless the wind is pushing on the sail from far abaft the beam), so you can shape that airfoil for the wind conditions. In light air, you want to get more draft in the sail in the appropriate location. Draft is the amount of deflection perpendicular to the boom in the horizontal direction. The proper shape will give you more power in light air conditions. Easing the outhaul will allow the sail to have this extra draft.
In heavy air, you wish to do the opposite. It''s called "blading out" the sail and requires higher tension on the outhaul to pull the sail tight along the foot, and extra halyard (and/or cunningham) tension to tension the luff. The latter not only helps reduce draft, but moves the draft more toward the mast, which is better than having the maximum draft near the middle of the sail. With the sail shaped like this (less draft) it will be depowered and reduce heel.
Another neat trick can be used on fractional rigs
. That''s where the headstay is attached lower than the top of the mast (say 7/8''s of the way up, hence "fractional"). This arrangement allows you to apply varying tension to the backstay (attached at the masthead), which actually bends the mast so that it curves forward near the middle. In the beginning, it wasn''t easy for me to visualize how this helps, but pulling the center of the mast forward definitely flattens the mainsail (reduces draft) and depowers it for heavy air conditions.
Hope this helps. If I didn''t get that quite right, someone else will chime in. Good luck with your learning. It''s all fun, isn''t it?