How many modifier combinations can your thumb hit? Each modifier combination gives your layout another set of bindings.

The Core Layout that Starcraft II players developed repositions your hand on the keyboard so that your thumb can better reach the modifiers. By moving your keyboard hand to a different position and, they setup the thumb to be able to do alt, control, shift, and control+shift which gives each key 4 bindings.

theCore homepage has decent videos explaining things: http://www.teamliquid.net/forum/view...opic_id=341878

I like some of the older versions of the videos a bit better for the way they show things
Why the Core? http://www.youtube.com/watch?v=I4luxRv4ye0
Positioning http://www.youtube.com/watch?v=gj5Xj9LJKZQ also http://youtu.be/zbI1YMEXz98?t=3m40s