There are many smartwatches that are water-resistant, but rarely is anything fully waterproof. That said, you can safely swim (in freshwater sources) with one that meets an IPX8, IP68 (or higher) rating. But there’s still risk of damage if the watch isn’t dried out after submersion, which is why features like the Apple Watch’s water lock are really handy. With water lock activated, the Apple Watch not only disables the screen so pressure from water doesn’t inadvertently trigger an unintended action, but it also ejects water from inside the device to help it dry faster.

It’s a useful and underrated feature of Apple Watch that every smartwatch should have, especially those that are rated to be safe to wear while swimming. I use this feature often with my Apple Watch Series 11, and I love that it’s automatically triggered any time I hop in a pool or wade in the ocean. It works while scuba diving as well if you have the Apple Watch Ultra, Apple Watch Ultra 2 or Apple Watch Ultra 3, all three of which are safe for deep diving (standard Apple Watches are not).

To manually turn on water lock, which is available on all Apple Watch models from Series 2 and onward, press the side button to open the Control Center. Select the water droplet icon (you may have to scroll down to see it). When the water lock is active, you won’t be able to manipulate the watch screen. It simply won’t respond. Once you come out of the water, press and hold the Digital Crown to unlock the watch. You will see a water ejection mode begin and hear a series of tones, which sound sort of like an ’80s video game, until the process is done.

What’s happening inside the watch when you do this? It’s pretty fascinating. When you press and hold the button, water comes streaming out of the tiny side speaker holes. It’s a lot more water than you might realize or even notice, since it happens so fast and the speakers and water droplets are so tiny. The watch pauses to settle and eject again, repeating this cycle over and over. This is why you have to press and hold the Crown for at least a few seconds instead of a quick press. It keeps you from initiating this process accidentally.

When should you use water lock? It’s a no-brainer to use the water lock feature on Apple Watch if you’re a swimmer or have a pool at home. In fact, you won’t have a choice, since it turns on automatically when you’re engaged in a water exercise like swimming or surfing. As an extra precaution, you might want to turn it on manually using the steps noted above during times when your wrist isn’t completely submerged, but the watch still gets wet. This might be if you wear it in the shower, while washing dishes or just during a run home in the rain. Utilizing this feature will ensure that any residual water is removed.

What happens if you don’t turn on water lock? The watch will likely be fine (assuming it’s a model with a high water-resistance rating), but you run the risk of water pooling inside and causing eventual damage. While that water-resistance rating gives you the confidence to wear an Apple Watch while swimming or doing other water-based activities, you should be aware that water-resistance weakens over time with any device. Keeping as much water out of the insides as possible, especially if you’re swimming in a chlorinated pool, can help prolong the life of your smartwatch. Next time you go for a swim, see if you can spot the water coming out of the side speaker when you press the button. And consider the intricate process that’s happening to keep the watch free of moisture inside.