One aspect of the jailbreak scene that always seemed like black magic to me, though, was the process of jailbreaking itself. The prospect is pretty remarkable: take any off-the-shelf iPhone, then enact obscene rituals and recite eldritch incantations until the shackles drop away. The OS will now allow you to run any code you point at it, irrespective of whether the code has gone through Apple’s blessed signing process, and paving the way for industrious tweak developers like myself.
Source: Exploiting the iPhone 4, Part 1: Gaining Entry | Phillip Tennen