Get one person to stand on a childs roundabout and another to try to push him around whilst standing on a skate board. Equal and opposite reaction will mean that the roundabout moves one way and the chap on the skateboard moves the other.
The roundabout represents the gears of the MRGearbox attached to the main rotor head and the guy on the skate board is the engine bolted to the fuselagetrying to drive the gearbox gears. The engine can't move but is attached to the fuselage that can and so it spins in the opposite direction to the rotor.
A simplistic explanation but I hope it helps.