We report an experimental study about the control of the turbulent wake of a square cylinder. The control device consists in a couple of compliant flaps implemented on the sides of the square cylinder. Both flaps are hinged so that they can freely rotate around their leading edge. Particular emphasis is given to the analysis of the wake through the investigation of both mean and turbulent velocity fields. The contribution to drag reduction of each of these physical parameters is evaluated from experimental data. The main conclusions are twofold: i. the control essentially modifies the turbulent velocity field leading to a reduction of the lateral flow entrainment, ii. this change is due to the synchronization of the vortex shedding on each side of the square cylinder.