Refer to figure.
Flutter is a periodic aeroelastic process. It is a self-induced oscillation, at which different oscillations are coupled via aerodynamic and inertial forces. Energy for the oscillation is provided by the airstream with velocity (v), the "channel" through which the energy is transmitted is lift (changes in lift) and used by the flexible system (having mass and elasticity).

If the CG is in the hinge, wing bending - controls surface flutter will not be started. Therefore, CG of the control is moved towards the hinge by the means of mass and balance by adding an extra weight in front of the torsion axis of the wing.