Heavy weather sailing
I just spotted the end of your post, in lighter air you don''t want to tighten the leach. In lighter winds, gradiant wind effects typically require a lot of twist in your sails. This means slightly loosening the vang, halyards, outhaul and mainsheet and bringing the traveller car well to windward. The idea is to get the air flowing across the sail so that the leech teletales on the mainsail are flying most of the time.
The reason that you did not see these suggestions in books are that they are exactly backwards of proper sail trim. The heavy weather advise does have a historic precident. Before boom vangs, adjustable travellers and low stretch sail materials, sails were often twisted open in heavy air and it was sometimes called a ''fisherman''s reef'', but as mentioned above this is a relatively poor way to reduce heeling and balance the helm, especially when cracked off a little.