Anything that vibrates (an oscillator) has a natural frequency. A vibration is basically a wave of energy – like a sine wave – and the speed that the wave changes direction from up to down through a full cycle is its frequency. Here’s the key: when the natural frequencies of oscillators are the same, it’s easy to transfer energy between them.
If you have two guitars sitting in the same room and they’re tuned alike, when you pluck, say, the A string on one, the A string on the other guitar will vibrate. This is called acoustic resonance. The energy of the sound wave generated by the first guitar travels through the air, contacts the second guitar and since both strings are tuned to the same natural frequency, the second one will vibrate too.
Another kind of resonance – mechanical resonance – shows up in many ways. When you push a child on a swing, if you time your push in phase with the swing, they will go higher – that’s because of the natural resonance of the pendulum. Breaking a crystal glass by exposing it to a specific sound pitch, the natural resonance of the glass, is another. And engineers have to pay strict attention to the natural resonance of buildings and bridges as part of their design process. If they don’t the structure they have designed can shatter – just like the glass – if it happens to be exposed to its natural resonant frequency from outside forces such as wind or earthquakes.
So how does it feel when you resonate with someone? How does it feel when you’re resisting them? And can you adjust your frequency (like that of the guitar) to resonate with someone better when you’re trying to understand them?