This is called synchronous antiphase. The motor, due to inertia, averages out the DC component of the square waves sent to it. If the positive part of the wave is greater than the negative part, the motor develops torque in one direction. The big plus of this scheme is that there is no dead spot near the servo null position, as you might get with some sort of sign/magnitude drive scheme. However, it does cause a lot of heating in the motor windings and armature iron, as well as the power transistors. Many commercial servo drives needed to add a series inductor in one of the motor wires, to combat this. Also, lots of potential for EMI.
Jon