Refer to figure.
Parasite drag describes the drag caused by the non-lifting components of the helicopter, most notably the fuselage and its components and as forward speed increases, the parasite drag increases too.

When the pilot applies forward cyclic to transition to forward flight, the rotor disc will be tilted forward and will produce a horizontal component of thrust.

This force acts at the rotor head, which is at some distance from the Centre of Gravity, thus a significant moment is formed which starts to pitch the fuselage nose-down.

As the fuselage begins to gather speed, the fuselage continues to pitch its nose down about the hub and begins to generate a small amount of parasite drag and the CG from being below the hub, now starts to move aft of the rotor shaft axis.

These two events cause two couples to be formed:

• Horizontal thrust couples with parasite drag and continues to pitch the nose down.
• Vertical thrust couples with the weight and pitch the nose up.

Initially, the horizontal thrust-drag couple does not provide enough opposition to arrest the nose-down pitching moment.

As speed increases, a point is reached where the combined effect of more parasite drag plus further aft movement of the CG provides a big enough couple to exactly counterbalance the vertical thrust-weight couple. At this point, the fuselage settles into a stable nose-down position.