Gaining access to an IPhone

Now, this question is for academic purposes only. Please do not take it anywhere else, that's not what it's meant for.

Recently I witnessed how ITunes, which is linked to a certain IPhone may unlock it's first screen - the one which requires a password.

I also know of a few tools which may mimic this effect;

I`m curious about how this mechanisem works. Does apple hold some sort of a "master key" that can be transferred via a USB device? Is that a part of the operating system, ITunes or some of those other tools? Is that a known weakness, or a feature? How do those tools (generally) work?