The theory as I was taught is that articulated systems will diverge at some frequency as the blades 'hunt'. It's usually not noticed because it's transient and low energy anywhere except at operational NR. The airframe is tuned away from most energetic and maximum exposure time, flight NR. That's why the AStar has heel springs, to change the airframe's natural frequency to keep it from being a harmonic to the NR. There are also dampers in the system (forward crosstubes on AS350) to absorb energy and keep it from magnifying the issue. Everything has to be within parameters, from the disk to the ground and the the heel springs should have the actual skid heels clear, a reason to takeoff again and perhaps move and try another SMOOTHER landing so you don't bump a blade out of phase and clear the heel allowing the heel springs to do their thing.
If you stop the process before anything is damaged, a change in the NR arrests the divergence because the excitation is not sympathetic/harmonic. That's a trap to the unwary- you can't accelerate the rotor fast enough from ground to flight idle to keep the airframe intact.