How do USB charging and "smart" charging ports (e.g. Anker's PowerIQ) work?
There are two additions to the USB specification that allow for more than 500mA current.

usb battery charging specification 1.1. Allows for up to 1.3A.

usb battery charging specification 1.2 (and this). Allows for up to 5A.

Summary:


USB 2.0 - BCS 1.1: 1.3A current, no data transmission.
USB 2.0 - BCS 1.2: 5A current with data.
USB 3.0 - BCS 1.2: 5A current handling but current limited to 1.5A, no data.


More can be found here

Edit: Sometimes the manufacturer doesn't follow the standard and uses something proprietary. Sometimes the "something proprietary" would be to just let the data pins hanging in the air or at a certain voltage and then provide an arbitrary amount of current...