Leeward and Windward A boat’s leeward side is the side that is or, when she is head to wind, was away from the wind. However, when sailing by the lee or directly downwind, her leeward side is the side on which her mainsail lies. The other side is her windward side. When two boats on the same tack overlap, the one on the leeward side of the other is the leeward boat. The other is the windward boat.
First of all leeward and windward have been in the rules from the almost the very first draft. They are both words in use since the wind was harnessed to propel a boat. Every sailor is well aware of how the wind hits his/her boat. It is the force behind propulsion.
Physically there are two positions a boat can have were the sides change; one at the bow pointing to the wind and one at the stern pointing away from the wind. The first one - at the bow - is sharply defined; your leeward side doesn't change until you have passed head to wind. As long as you stay head to wind your leeward and windward side stay as they were before you luffed. The latter position is not so sharply defined. There it only changes after your sail has gone to the other side. This means that the wind can blow from the same side as your sail is. (sailing by the lee)
Pushing the sail to the other side doesn't change your leeward or windward side. Because you are using an external force (with your arm/body) it would not be the side a sail "naturally lies". Same is true when you trim your sail midship. The side where your sail would go if released, is still the leeward side.
A leeward or windward boat always has an overlap, but a boat with an overlap isn't necessarily a leeward or windward boat. You can have an overlap with a boat on a different tack when rule 18 is applicable. Normally if boats are not overlapped, they are clear ahead and clear astern.
A boat is on the tack, starboard or port, corresponding to her windward side in all but two countries in the world. Only in the Netherlands and in Belgium (Flanders) a boat is on the tack - stuurboord (starboard) or bakboord (port) -, corresponding to her leeward side. We pigheaded Dutch refuse to change it.....
We do this because we want everybody to stay on their toes when yelled "Port" at. You are then to check if it is a Dutch boat, and if so they might be on starboard tack.