This was an exploit that allowed the running of unsigned code at iBoot level, present in pre-2.0 versions of iBoot.
This is a very easy-to-use exploit. In earlier iBoots, if a parameter was given to the 'diags' command, then it would jump to whatever address argv specified, but not before disabling the GPIO devices. You can run unsigned code on the S5L8900 using this, but the GPIOs need to be restored if you intend to use any I/O again (such as the screen, serial, or USB).
In 2.0 iBoots, they have a flag check on this command (checks bit 4 of the iBoot flags), and that flag will not be present on a retail device, just an engineering one with a 'whitelisted' CHIPID, so this exploit doesn't work. However, the 1.1.4 iBSS can still be used on the iPhone, iPhone 3G, or iPod touch 1G, and can be used to boot a modified 2.0+ iBoot. Therefore, it is still an open exploit for these devices.