QUESTION: How does a Rudder/Skeg Combination help create strong directional stability?
ANSWER: A large full length skeg (which is structural) directly attached to a hull, is analogous to feathers attached to an arrow shaft. Both act as stabilizers helping to maintain a consistent and true course.

Directional Stability Increased with Full Skeg

From Yacht Designed Explained by Steve Killing, Douglas Hunter
             “The skeg, which sits solidly in front of the rudder, performs two functions-- [...] The real benefit of a skeg rudder is the increased tendency of the boat to travel in a straight line. Because the skeg is like a fixed rudder on centerline, it helps maintain a straight course in lumpy waves, with the rudder itself used only for course corrections.”
From “Offshore Yachts” by John Rousmaniere
               “Second, directional stability is improved by the skeg because tit acts like tail feathers well aft of the center of gravity. The dorsal-type fin forward of many skeg-mounted rudders also acts as a stabilizing factor [...].”