when the C of G goes aft of the center of lift (for lack of better term), the elevator will need to produce an upward balancing force. therefore, part of the weight being supported by the wing is reduced. in effect, you have slightly 'lighter' airplane.
for the same reason, the stall speed also reduces with an aft C of G.
however, the flight stability of the airplane degrades as the C of G moves aft.
SR