Letting physics do the work
Not every motion needs to be commanded by a motor. A system's passive dynamics are the motions it naturally produces on its own, from gravity, inertia, and momentum, with no actuation at all. A swinging pendulum released from an angle will keep oscillating for a while purely from gravity and its own momentum. Underactuated control often works best not by overriding these natural dynamics but by cooperating with them, applying small, well-timed inputs that nudge a naturally occurring motion rather than forcing an entirely different one.
This is a real shift in mindset from fully-actuated control, where the goal is usually to command an exact trajectory regardless of what the system would do on its own. In underactuated systems, fighting the passive dynamics head-on is often expensive or outright infeasible given the available actuation, while working with them can achieve the same goal using far less control effort.