Lonewolf,
Peter Batten and Richard Woodward of Coax Helicopters have spoken about this method of yaw control on several occasions. For example in
this interview on Mick Cullen's Rotary Wing Show.
It is mentioned in the written summary available at the hyperlink above:
"Yaw control in coaxial rotor designs can be problematic but is overcome in Coax Helicopter’s designs by drag flaps on the end of the rotor blades that provide a torque differential between the blades."
Or go to 26' 20" in the podcast available under this link and hear Richard talk about it.