The lag hinge in an articulated rotor system is there to alleviate the high chordwise bending stresses that would otherwise occur due to flapping.
In an articulated rotor system, blades are allowed to flap about a hinge to alleviate the high stresses and hub moments induced in forward flight due to the asymmetry of lift. This flapping motion takes place in a rotating coordinate frame. Just as winds and ocean currents are deflected on the spinning earth, so flapping blades in a spinning rotor experience a Coriolis acceleration at right angles to the motion.
After all, the equations of motion are the same--Newton didn't set up a special case for helicopters...
You can also picture it if you realise that as a blade flaps, it changes its distance from the rotation center by a bit. Like the spinning ice skater who pulls in her arms to speed up, the blade "gets ahead of itself." The situation is reversed when the blade flaps downwards.
The result is that the blade tip wants to describe a small ellipse about a radial drawn from the center. This has a few not-so-obvious implications. For example, because blades can move in-plane, the center of mass does not always coincide with the center of rotation, even for a perfectly balanced set of blades.
Some rotor systems do without a lag hinge. So-called rigid rotors use flexible sections to create "virtual" hinges.
Teetering rotors use undersling to alleviate in-plane stresses. The flapping hinge is displaced upwards from the blade axes. As the rotor flaps, the joint offset causes the descending blade to move *inwards* while the ascending blade moves outwards. These motions generate Coriolis accelerations of their own directed opposite to the forces generated by the flapping itself.
