If Steam can map the guide button on the Xbox 360 controller why can't I

Question

I want to map the Xbox Guide button on the Xbox 360 controller.

I've been searching for a possible solution for some time now and from the looks of my google results it seems that the only way to do so is by using a custom driver.

However, I tried using Steam Big Picture mode today and was surprised to see that Valve has managed to achieve this to open the Steam Overlay. So how come I can't find any information on how to do this? Does anyone have any idea of how they've done it?

I've thought of possible solutions, aside from a custom driver, and I wonder if you could read the raw data from the controller? The button must send a signal so it should be possible detect when it's pressed, right?

Any help is much appreciated.

Answer 1

To mark as answered: @luke answered this in the comments with:

Assuming you are using XInput, it appears that use of the Guide button is undocumented. However, people have figured out that it apparently sets bit 0x0400 in the button bit flag. More information.