The basic premise is this:

At high pressures, your tires bounce over every imperfection in the trail surface. Your energy is used for UPWARD movement, instead of FORWARD movement.

At lower pressures, your tires are able to deform over trail imperfections (rocks, roots, gravel), so all of your energy goes towards going FORWARD, instead of propelling you over every little bump.

This is just one link, but there are many. If you were to look at WC MTB tires over the years, they've become wider and wider. Dudes aren't running 1.5" tires anymore.

http://www.schwalbetires.com/wider_faster_page