Wouter,

are you sailing trough tacks with a tight leech? I think not.. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />
In other words, you will release rather a lot of sheet tension in tacks. Loads on the luff, especially in superlight conditions where the downhaul is really cranked on, will be rather large no matter what you do. I am of the opinion that the luff should be able to take downhaul loads even if you release the mainsheet. Yes, I know what Glenn has said earlier, and I dont consider it good practice going downwind with downhaul even if we used to do so on the old rig Tornado in strong winds...

I had an "eye opening experience" when a former olympic Tornado sailor was testing how flat he could make our new mainsail. Mainsails are tough sails taking very high loads!