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This exploit is in the Baseband Bootrom. There are hardware (testpoint) and software variations of this.

Credit
gray, iProof, geohot, dinopio, lazyc0der, and an anonymous contributor

Description
If 0xA0000030 0xA000A5A0 0xA0015C58 0xA0017370 read as 0xFFFFFFFF on startup, the Baseband Bootrom Protocol can be used to download and run unsigned code. In the initial hardware unlock, an address line was pulled high to OR in hardware those addresses with 0x40000.

Other links
dev team description of fakeblank