Battle Sails is a technique that increases your Sail Defense by 15; therefore, any opponent aiming at a ship using this technique will lose Accuracy by 15%. Meaning, Battle Sails is only an effective technique if your opponent is using a Dismantling Shot (i.e., any shot that does damage primarily to an opponent's rigging: Star Shot , Chain Shot , or Bar Shot ).
To set Battle Sails, simply adjust your ship's sails to 75% using the Ship Speed Slider (which is located on the Ship Display) or simply type B. Obviously, this technique will lower the overall speed of your ship, and so it may be the wrong technique to use if speed is important.
The Battle Sails technique also increases Accuracy for the ship applying it. This increase in Accuracy is really a side effect. Higher speeds will negatively affect a ship's Accuracy, so by lowering the speed (adjusting your sails to 75%), you are increasing the ship's Accuracy.
Battle Sails are not as important a technique for Warships. This technique is not as important because the speed of a Warship will usually be low enough that the slower speed caused by Battle Sails won't improve Accuracy too drastically. However if you are sailing a Warship, you should enable Battle Sails if you expect Dismantling Shot—the Battle Sails technique will help protect your sails.