For those passages where you want a kind of blurry, soft, and dreamy sound (lots of those in Debussy, Ravel and Scriabin) there are two things which are technically important. Keeping most of the weight off your hand--- almost as if your wrist is limp--- and playing every note in one flowing motion.
Imagine that, instead of a row of keys, your keyboard is a stream of water; with the first note of the passage, you dip your hand into the water, and in one smooth arc, you scoop some of the water up and out. I like to play the highest notes in these kinds of passages a bit brighter. Maintaining the same mental image, you can give the higher note a nice bell-like tone by flicking some of that "water" at the audience.
It's not literally the same movement, but that mental image helps me explain the way it should be played.