It's all about power level vs sail plan drag-related efficiency;

Phase one of depowering is use the downhaul and don't touch the mainsheet. Once the wind / gusts get beyond where the downhaul can manage it, it's time to start working the mainsheet and average the downhaul position to a manageable level (unless the two of you are incredibly in sync and can work both the main and the downhaul in sequential concert without trading for an unsteady helm).

Once the crew starts to be able to anticipate the puffs with the downhaul, you'll start to feel the boat make little leaps forward every time you hit it just right. That magic happens when a puff hits, the helm doesn't change much at all, the boat heal remains the same, and speed increases through the puff. If anything, you can sail slightly higher at this moment if you need the height. If the crew can then ease the downhaul and power back up before splashdown - you're hauling.

Besides the obvious of flattening the mainsail and moving the draft forward a little, the downhaul (more importantly, in my opinion) lets the head of the sail twist off while still permitting the mainsheet to maintain tight leech tension. Downhaul reduces power in the sail, makes the center of effort lower (now that the head is not producing as much and the bottom is still sheeted tightly to center) but efficiency increases, due to the slightly flatter shape, the twist in the main, and the maintained leech tension. If you are just easing the mainsheet and not playing the downhaul, you're depowering but not reducing drag.


I don't know why monohulls do what they do but a lot of the way we use downhaul has to do with square head mains most of us are running...and so few monohulls use fat heads, it doesn't really relate.