Leading and lagging is a capability designed into a fully-articulated rotor system to reduce the stress on the rotor system due to blade flapping. The need to lead-lag is due to the Coriolis effect, otherwise known as the law of conservation of angular momentum. As a rotor blade flaps up, the blade’s speed increases because the center of mass of that blade moves closer to the axis of rotation. As the blade flaps downs, the center of mass moves away from the axis of rotation and the speed of that blade slows downs. The lead-lag hinge allows the forces to equalize, which removes undue stress on the system. Lead-lag may also be referred to as hunting or dragging.
With a semi-rigid rotor system, such as on the Robinson R22/R44, there is no vertical drag hinge as the design minimizes any impact from the Coriolis force. Due to the underslung hinge, the blade moves outward when it flaps up, so the center of mass of that blade does not change significantly. Any remaining lead-lag forces are absorbed through the blades.
A ridged rotor system does not have flapping or lead-lag hinges. With a ridged rotor system, these forces are absorbed through bending of the blades.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 4-4
Principles of Helicopter Flight, 2nd Edition, pg. 78
FM 3-04.203-2007 Fundamentals of Flight pg. 1-15