Underwater appendages, like keels and rudders generate lift as water moves over them—which helps prevent leeway. This is particularly true of more modern fin keels and spade rudders, rather than older full keel designs. Keeping the boat's sail plan well balanced, with just a bit of weather helm allows the rudder to stay in the range where it is generating lift to windward instead of just generating drag. If the boat is unbalanced, either too much weather helm or lee helm, then the rudder doesn't generate lift as designed, and the boat's performance suffers.