As the headsail, staysail, and mainsail are trimmed in, the gap (slot) between them narrows, and they begin to affect each other. In lighter conditions (under 10-12 knots apparent), the staysail may inhibit, not help, upwind performance. In the middle ranges, the staysail can be more helpful, depending on factors like sail shape and sheeting angle. In windy conditions, as the mainsail has to be eased, lowering the staysail will allow the mainsail to breathe and reduce back-winding, helping to keep the boat on its feet. In short, the optimum conditions for the staysail (when adding horsepower), are at apparent wind angles of 50-130 with a minimum apparent wind velocity of 10-12 knots.