The basic idea of this trick is that you can set up your iPhone to always be in a “listening” state. From there, you can have the audio picked up by your iPhone and transmitted right back to your AirPods. (This iPhone trick also works with regular AirPods, Powerbeats, and AirPods Max.)
To be perfectly clear, it's only natural to see this iPhone hack in the context of spying on other peoples' conversations. Still, it's worth bringing to your attention as something of a safety warning - for example, it you see a random iPhone just laying around, don't assume it's safe to talk about private things!
Apple describes the purpose of this feature as follows: With Live Listen, your iPhone, iPad, or iPod touch can act like a microphone that sends sound to your AirPods, AirPods Pro, AirPods Max, Powerbeats Pro, or Beats Fit Pro. Live Listen can help you hear a conversation in a noisy area or even hear someone speaking across the room.
To get started with this iPhone hack, make sure your AirPods are out and active. Next, select Settings > Control Center > Customize Controls. Once you do this, you'll see a list of apps that reside in the Control Center. You'll also see apps that aren't in Control Center. Keep on scrolling and you'll see an option that saysHearing. Once you select that, hit the green plus icon whereupon it will now appear in the Control Center. Following that, you'll see an ear icon at the bottom of the screen when you call Control Center into action by swiping down from the top of the screen. If you tap the ear icon, you'll see an option that reads “Live Listen.” Tap that once and any sound the iPhone picks up will be sent back to your AirPods if you're in range. While this could be great for journalists trying to follow a press conference from the way back of the room, it may seem fishy for something you find at a cafe or cofee shop.
Here's the original post that exposed the "hack":