Heavy weather sailing
I really think that that quote is referring to a very subtle adjustment that makes sense for race boats in a very narrow band of conditions. For cruisers which cannot burn off speed by planning or surfing when they are over powered and where sail stretch happens automatically, there would be few instances where where easing the vang makes sense.
On most boats with dacron sails, there is sufficient stretch in the sail that the sail automatically increases in draft and twist. But on boats with lower stretch high tech fabrics you can more effectively use twist to depower the boat a little without powering the sail up. As he says, it is a secindary adjustment. I don''t think it is very effective compared to leaving the vang quite tight and allowing the traveller and the sheet be eases to change incident angle.
This is an easy thing to experiment with and try out for yourself. If you go out on a breezy day with another person, take turns steering. Set the jib for the conditions and then steer by the jib teletales. Have the other person try bringing the traveller to windward a bit and easing the mainsail to the point where it is about to luff and feel the helm. Then ease the traveller and bring the mainsheet in tighter and feel that. Go through a number of cycles and feel the difference. Similarly put the boat on a beam reach and try easing and tightening the vbang while adjusting the mainsheet so that the sail is not luffing. At least on a tiller steered boat or a boat with minimal friction in the steering system the loads should be obvious. Even with a boat that has wheel steering you should be able to see the amount that the wheel is turned.