It's a feature Dinnerbone put in last month. The falling from... I'm forgetting, so I'll guess 3-4+ blocks makes a puff of whatever you landed on, which is bigger the higher you fall.
It isn't perfect though. Seems if you are in Creative, fly down really far, then fly back up and land, it makes the effect despite landing slowly. It's just counting blocks downward and not inverting them up.