AirDrop: Bluetooth or Wi-Fi? How does AirDrop work? I thought it is just Bluetooth, but I am starting to doubt that now.

Whenever I turn AirDrop on, my Wi-Fi and Bluetooth are automatically turned on. If I turn the Wi-Fi off, and try to AirDrop, sometimes it works, but sometimes it doesn't. It never works if I turn the Bluetooth off instead.

So... Why does this happen? How does AirDrop work?
AirDrop uses Bluetooth to create a peer-to-peer Wi-Fi network between the devices.

Each device creates a firewall around the connection and files are sent encrypted, which actually makes it safer than transferring via email. AirDrop will automatically detect nearby supported devices, and the devices only need to be close enough to establish a good Wi-Fi connection, making it possible to share files across several rooms.