Look at (or visualize) a 2-bladed rotor system. It is known as a "teetering" system because the rotor hub is attached to the rotor mast at a single point - the teeter hinge - and it is free to pivot about that point to allow the blades to flap. If you look at a helicopter from the front, with the blades running laterally (like the wings of an airplane), if you pushed the left blade up, the right blade would go down, just like a teeter-totter.
A design feature of teetering rotor systems is that the hub is "underslung", meaning the rotor blades' plane of rotation is
below the pivot point when viewed from the side. Imagine a steel rod with a hole through one end standing vertically (rotor mast). Now make a steel box with a corresponding hole through the sides. Put the box over the rod, put a bolt through the holes, one model teetering rotorhub!
Rotor viewed from top:
blade ........... rotor hub .......... blade
============[()]============
teeter hinge axis is up/down in this diagram,
running between the ellipses (rotor mast)
Obviously, the hub can only have a certain range of teetering before the inside of the hub will hit the rotor mast. In normal flying, the only time these limits might be approached is in a slope landing, where the fuselage and rotor mast are at an angle while the rotor disk remains perpendicular to "true vertical".
However, in a low-G condition inflight, the angle between the rotor disk and the airframe can very rapidly exceed the flapping limit of the rotor hub. The hub hits the mast and in short order, the mast can separate. Bad thing.
There's a lot more aerodynamics, geometry, and physics involved, but that's the gist. For a little more info on heli-flight overall, may I suggest the
Dynamic Flight website
John