Wi-Fi Assist, a new feature in iOS 9, was initially met with some skepticism as Apple did a lousy job detailing how it works beyond stating that Wi-Fi Assist automatically uses cellular data when Wi-Fi connectivity is poor.
“With Wi-Fi Assist, you can stay connected to the Internet even if you have a poor Wi-Fi connection,” the iPhone maker said somewhat critically.
However, inexperienced users might encounter some unwanted effects when using their devices in areas with low Wi-Fi signal because Wi-Fi Assist will activate their carrier's mobile data network and therefore possibly lead to increased data consumption and unwanted overage fees on the wireless bill.
Here's what Apple is saying how Wi-Fi exactly works.