Blue line is like speed, histogram - like acceleration. So if blue line is >0, price going up, if histogram is above zero (i.e. green) speed is going up. Change from red to green is inflection point, switch from dexeleration to acceleration. The indicator is trailing, so it shows averaged price movement some time ago, but assuming price movements are oscillating and have inertia, it has some predictive power.
 https://bitcointalk.org/index.php?topic=499968.msg7386834#msg7386834