In a nutshell, the blades are wings, and when they are set to critical angle of attack, they stall. Just like a wing, the blades pitch downward at stall, due to the lift shifting aft on the airfoil as the smooth flow breaks up. This stall can be caused by strong maneuvering, high gross weight, high altitude, low rotor rpm, in fact, all the factors they taught you in school.
On a helicopter, the servos are all that stands between that blade downward pitching and your cyclic stick. Follow the forces on the diagram in the UK publication that puntosaurus pointed to, you can see how the blade tucks in a downward feathering motion, the pitch link resists, and passes the load to the swashplate, which then passes the load to the servo. On some model helicopters - very few- the servos are not designed to take all the force the blade can give out. This means that even at maximum servo force, the blade can push the servo backwards and shove the cyclic in your hands on these few helicopters. Sometimes, the servo has a switch in it that compresses when the servo is about to be driven backwards. This light is a warning to cool down your jets and reduce the maneuvering.
This servo back drive is called many things - Jack Stall, servo transparency, servo stall, control feedback. Few helos have the servo stall propensity, but those that do can surprise the pilot, so read and heed.