A coaxial helicopter has two main rotors. The helicopter will rotate if one of the two motors/rotors spins more slowly or faster than the other. By changing the speed of one rotor you will generate a torque, which will make the helicopter rotate. The torque of a hovering helicopter is zero. Thus, if one of the two rotors spins faster than the other, the helicopter will rotate in the same direction like the faster spinning rotor. In simple fixed pitch coaxial helicopters the horizontal tail rotor is responsible for forward and backward movements.
Single Blade Helicopters use a vertical tail rotor to prevent uncontrolled spinning. The tail rotor pushes against the main rotor torque and keeps the model hovering. Using more or less thrust will rotate the helicopter towards another direction. Control the forward and backward movements by adjusting the align of the cockpit. If the nose comes down, the helicopter will move forward; lifting the nose up makes the helicopter flying backwards. Therefore this is also called nick. You can reach this by adjusting the swashplate of the main blades.
Increasing and decreasing the speed of the main blades is the easy way to ascend or descend. Fixed angle of the main rotors is called fixed pitch. Pilots of fixed pitch models must adjust the torque of the main blade manually or invest in a gyroscope, which will compensate the torque automatically. More advanced models adjust the angle of the main rotor blades to change the boost of the helicopter (collective pitch).