For a helicopter on the ground, as the blades rotate, they describe a horizontal disc due to centrifugal force.
However, as the helicopter generates lift, the blades are pulled upwards into a cone shape.
[4] The coning angle depends on RPM, gross weight and g-force.
[3] If rotor RPM drops too low, the centrifugal force will reduce until is no longer sufficient to balance the lift, and the rotor blades fold up with no chance of recovery.
Some helicopters such as the Bell UH-1 Iroquois are designed with "pre-coned" blades, which are curved downwards but lay more flat in flight.