The tailplane always has a stabilizing effect, since it is well behind the CG.
The amount of up or down tailplane deflection has an effect in the trim angle of attack of the airplane. This deflection has to be such that the THS balances all the moments about the CG due to the wing and fuselage lift plus those due to the wing aerodynamic moment. The final result can either be an ANU or an AND position, depending on the CG, for a given weight and AoA.