The boat will want to turn around your leg. Once the boat is heeled the rudder becomes less efficient. Rudders are generally designed to rotate the stern of your boat back and forth within a plane perpendicular to the rudder. This works relatively well when the boat is standing upright, but when it is heeled some of the rotational force wants to push the bow up or down rather than sideways. The wider the boat the greater the effect.
Lightnings actually use their chines to create some lateral resistance.
When the chine hits the water it acts like a bent ski. The boat wants to follow the curve that the chine makes as it curves from bow to beam. Lightning sailors quickly learn that there is a critical heel needed to sail well upwind. The hard chines and flat bottom of a Lightning make the boat very fast downwind in planing conditions, but they can be very challenging if you happen to dip a chine at high speed. Make sure the boat is sailed flat and in many different conditions before you assume that an alteration in your sails will solve the performance problem.