Apple this week released a new version of iOS that includes a feature that makes life much more difficult for anyone who tries to download the contents of a device, whether that be a thief, an attacker, or a forensics investigator.

The feature has been in several beta versions of iOS over the last few months, hiding slightly below the surface. It’s called USB Restricted Mode and Apple inserted it into iOS 11.4.1, which was released Monday. The new mode has a very simple function: to prevent USB accessories from communicating with an iOS device that hasn’t been unlocked in at least an hour. That may seem like a nuisance for normal daily use, but the feature plays a specific and important security role.

When thieves (or attackers who have physical access to a device) get hold of an iPhone or iPad, one of the things they will do quickly is try to dump the data from the device. There are a number of ways to do that, most of which involve connecting the device to a computer or specialized forensics device. Those connections happen through the iPhone’s USB Lightning port, and Apple’s change can prevent thieves or other attackers from being able to get access to the device’s contents. Restricted Mode is enabled by default in the new version of iOS.

“Starting with iOS 11.4.1, if you use USB accessories with your iPhone, iPad, or iPod touch, or if you connect your device to a Mac or PC, you might need to unlock your device for it to recognize and use the accessory. Your accessory then remains connected, even if your device is subsequently locked,” Apple said.

“If you don’t first unlock your password-protected iOS device—or you haven’t unlocked and connected it to a USB accessory within the past hour—your iOS device won’t communicate with the accessory or computer, and in some cases, it might not charge. You might also see an alert asking you to unlock your device to use accessories.”