Fixed Pitch (FP) heli's do not change the pitch of the blades in order to climb or descend, Instead, they rely on changes in throttle (and therefore headspeed) for added or reduced lift to change altitude. Think of increasing or decreasing the throttle on an airplane engine to increase or decrease thrust.

Collective Pitch (CP) heli's will increase the pitch of the blades as the throttle increases while the headspeed remains constant. With the constant headspeed, the increase (or decrease) in blade pitch creates the change in lift needed to cause the heli to climb or descend.

Keith