Apple garnered a lot of criticism last month following the release of iOS 9, when users started seeing higher than normal data usage on their phone bills.

The blame was quickly placed on Wi-Fi Assist, a little-discussed feature buried at the bottom of your Wi-Fi settings that meant your phone wouldn’t try to cling to bad Wi-Fi signal.

Many tech sites suggested it be disabled, however Apple quietly added a support page over the weekend to explain just how the feature works, and it’s smarter than we thought.

In particular, the company emphasised three key areas:

Wi-Fi Assist will not automatically switch to cellular if you’re data roaming.

Wi-Fi Assist only works when you have apps running in the foreground and doesn’t activate with background downloading of content.

Wi-Fi Assist doesn’t activate with some third-party apps that stream audio or video, or download attachments, like an email app, as they might use large amounts of data.

Apple also said that you’ll see the cellular data icon alongside the Wi-Fi one when the feature is being used, but reiterated that the usage should only be a “small percentage” higher than you’ve seen in the past.

If you’ve disabled Wi-Fi Assist in the past and have updated to iOS 9.0.1, it’s worth considering turning it back on again.

➤ About Wi-Fi Assist [Apple, via 6Colors]

Read next: Yahoo Mail gets a massive overhaul with a new look... and no need to use a password