You may not know what a Shepard tone is, but you’d definitely recognize one if you heard it. This is especially true if you’ve seen any movie by Christopher Nolan and his longtime collaborator Hans Zimmer, the Hollywood composer whose style is imitated in just about every blockbuster action movie you’ve seen in the past two decades.

A Shepard tone is an illusion that makes the audio sound like a musical scale that’s infinitely rising. The explainer below from Vox goes into how Zimmer’s passion for Shepard tones achieves the tense feelings Nolan wants to evoke in his audience. You can hear it in the recently released Dunkirk, but whether or not you’ve seen the World War II film, you’ll recognize the effect. It could also be familiar to you from Super Mario 64’s endless stairs sequence or from the end of The Beatles song “I Am the Walrus.”

A Shepard tone works by layering multiple different tones that are separated by an octave each. The highest tone gets quieter as it goes up. The lowest tone gets louder as it goes up and the tone in the middle stays at the same volume. You always hear at least two of the tones rising, so to your brain it sounds like it’s one constant ascending tone. (Or descending.) As Vox’s Christophe Haubursin explains, “It’s like a barber’s pole, constantly seeming to rise without actually going anywhere. Put that in a soundtrack, and it creates the sound of rising tension that carries the screenplay forward.”

And it isn’t just used in scores. In the 2008 movie The Dark Knight—another Nolan/Zimmer film—a Shepard tone was used to create the acceleration sound effects for the Batpod. According to Richard King, the movie’s sound designer, “When played on a keyboard, it gives the illusion of greater and greater speed; the pod appears unstoppable.” As is your anxiety while listening.

This may be the one explainer video that will truly keep you on the edge of your seat.