Signal Quality. Spektrum telemetry modules keep tabs on the quality of the signal coming to the receiver and send this information back to the transmitter where it is represented on the LCD screen using a familiar signal bar scale.
A pendulum is a weight suspended from a pivot so that it can swing freely. When a pendulum is displaced sideways from its resting, equilibrium position, it is subject to a restoring force due to gravity that will accelerate it back toward the equilibrium position.
Simple harmonic motion: the swing of the pendulum. Nudge something away from where it was at rest, and often it will push back. And then overshoot. From atoms to bridges to wheeled luggage, much of the world is in simple harmonic motion.