The upper scissors forces the swashplate to turn with the rotor head, but can also hinge to accommodate control motions. Sometimes there are two rotating scissors to share the load, usually placed 180 degrees apart. Since the scissors is a rotating mass which must be balanced, this is a somewhat useful application of weight that would otherwise be required. The lower scissors keeps the stationary side of the swashplate in the proper orientation with respect to the aircraft and the controls, and usually occupies the position of the 'missing' control input. Because of the relatively simple kinematics of their linkage to the swashplate, the blades will be forced to change pitch as they rotate.
People often mistake helicopter control as tilting the rotor disc.
While this may make for simple explanation and visualization, it is more accurate to think of helicopter control as the tilting and lifting of the swashplate. Blades Originally, helicopter blades were fabricated from wood, with perhaps some aluminum bonded to them to form the aerodynamic section of a lighter material. Eventually, blade construction changed to metal spars with the aerodynamic section fabricated from metal, and later, from composites. Current technology uses blades fabricated almost completely of composites. The weights permit the designer to better tailor the blade's flight characteristics and control feedback forces.