For perfect trim on a reach, ease in every puff. Conversely, the sail will need to be trimmed in lulls. If the boat is overpowered in a puff, the sheet can be eased, allowing the sail to luff and spill excess power. Off the wind, the mainsheet simply moves the sail in and out of the boat, changing the sail’s angle to the wind. Upwind, as the sail nears the centerline of the boat, the mainsheet begins to pull down, affecting the twist.