I came up with this tweak while cross-refering WS&IM with the miniature rules Heart of Oak: In the Advance Rules for WS&IM, you are allowed to Back Sails whenever you like. Not only is this unrealistic, it also takes away much of the depth in naval tactics.
In the real world, a square-riggers (those having sails that are 90 degrees to the bowline) could Back Sail only when they are 'beating' or, in WS&IM terms, at attitude 'C' (bow is 60 degrees from the wind). By beating you are sacrificing speed for extra manoeuverability, this is especially important if you are trying to maintain a line of battle.
True historical decisions could not be appreciated if such a limitation is not implemented. In Trafalgar, the French fleet was beating into the wind before Nelson broke their line with the wind behind him. The French weren't stupid, they are just bound by good sense.