All of the quad-copters that I have seen have fixed pitch propellers. Two of the rotors turn clockwise and two turn counter-clockwise. In order to produce counter-clockwise yaw, the clockwise propellers will have to increase speed and the counter-clockwise propellers will have to decrease speed. Clockwise yaw will require the opposite. I suspect that some of the yaw is produced by the acceleration/deceleration forces on the rotor and some yaw is generated by the difference in torque required for the differential rotor speeds, but the end result is yaw in either case.
All multi-rotor craft share the same characteristics.