iREB didn't exist in a vacuum. It was part of a larger jailbreaking ecosystem:

Comprehensive lists of legacy iTunes error codes and their technical meanings.

Connect your iPhone, iPod Touch, or iPad to your computer via a USB cable. Enter Pwned DFU Mode Extract the ZIP file and run the executable as an administrator.

Connect the iOS device to a computer running Windows (iTunes 10 or later was typically required at the time).

Does not require active server communication with Apple to patch the local computer's iTunes instructions. Step-by-Step Guide: How to Use iREB

Its primary function is elegantly simple. iREB uploads specially crafted "pwned" files using a system called iTunnel. This process puts a connected iDevice into a special . In this state, iTunes is essentially tricked into accepting the custom firmware without the standard verification checks that cause the restore to fail.