In I and II, the helicopter is hovering in calm condition, the tip path plane is horizontal and the axis of rotation and the shaft axis are in line. In III and IV, however, forward cyclic has tilted the disc forward, but the shaft axis is still vertical. From overhead it can be seen the tip path plane has moved forward (as shown by the dotted circle), and the blade tip retain their respective opposite on the tip path circumference and their velocity remains constant. To achieve this, the advancing blade A must move forward on its lead-lag hinge and the retreating blade C must move aft, a phenomenon known as Hook's joint effect.
The movement of a rotor blade to reposition itself relative to the other blades when a cyclic stick is applied. This situation arises when the plane of the rotor disc and the drive shaft are not normal to each other.