Since 2013, we’ve been able to use handheld electronic devices such as the iPhone, iPad, and Kindle at pretty much all times during airplane flights, including takeoff and landing. That was a big change from previous policy, which banned the use of personal electronic devices below 10,000 feet, forcing passengers to occupy themselves with books and magazines at the start and end of flights.

But now flight attendants ask us to put our devices into “airplane mode.” You probably know how to do this on your iOS device, but if not, here’s how. Swipe up from the bottom of the screen to bring up Control Center and tap the airplane button at the top left. Alternatively, you can open the Settings app and enable the Airplane Mode switch (it’s the first switch in the list). When you land, use the same controls to turn it off again.

What does airplane mode do? It disables certain wireless features of your device. Specifically, it turns off the cellular voice and data features of your iPhone or iPad, and on all iOS devices it turns off both Wi-Fi and Bluetooth. However, only the cellular features are important to your airline—you can re-enable both Wi-Fi and Bluetooth at any time. That might be useful if you want to use the airplane’s Wi-Fi network for Internet access (usually for a fee) or Bluetooth to play music over wireless headphones.

To turn these wireless features back on, tap the grayed-out Wi-Fi and Bluetooth buttons in Control Center, or flip their switches in Settings > Wi-Fi and Settings > Bluetooth. Don’t bother turning them on unless you’re going to use them, though, since you’ll save a little battery life by leaving them off for the duration of a long flight.

Why do the airlines care about cellular? It has little to do with airplane safety; the prohibition originated from the U.S. Federal Communications Commission, not the Federal Aviation Administration. The reason is that fast-moving cell phones used high in the air may light up many cell towers at once, which can confuse the mobile phone network.