Resonant beam breaker
A resonant beam breaker is a pavement-breaking machine that drives a long steel beam at its natural bending frequency. Because of resonance, a modest vibrating force at one end builds into large, rapid blows from a hammer foot at the other end. It was used to crack old concrete and asphalt quickly.
Example: It works like flicking a long ruler hanging off a desk, where small taps at the right rhythm make the free end swing hard.