Lets start with the basics of the real thing compared with the common starter models.

A real helicopter has one rotor on top and a small one at the rear.
Why does it have a small one at the back?

Well as the top rotor rotates it creates a torque on the main body, which would make the body rotate almost at the same rate as the rotor. So to prevent this the tail rotor spins to keep the body from rotating... this rear rotor is used for steering while in the air.
If the rear rotor is slowed down slightly then the body will rotate with the main rotor, the slower the rear rotor spins the quicker the body rotates, also if the rear rotor is sped up then the helicopter will rotate the other way... once again the faster the rotor rotates the faster the helicopter will spin.

So why does an contra rotating model NOT have a rear rotor?
This is why the model has two main rotors....

On models with two rotors, which spin in opposite directions there is NO torque in the body like a real helicopter....
So how does it steer? You may ask.....

Well this is the clever bit on the models, to rotate the model helicopter one way or the other, then one set of rotors is slowed down.
As the rotors are contra-rotating, then which ever one is slowed down the model will rotate the other way...

On the models the lower set of blades is connected to two servos, which will tilt the blades slightly.
This is used to move around while in the air.
One servo is for forward and backwards and the other one will enable left and right movement.

I hope this helps some of you understand how it works