Notice how you must let the upper arm muscle release when the dart leaves your hand? You throw the weight of your arm with the dart upon its release. When you play big chords or play forte passages on the piano the muscles must learn to “let go” and release that weight into the keys in much the same way. “Letting go” is vital for the pianist who wants to produce a beautiful “tone” (sound) at the piano.
For examples of exercises that help with this approach on the keys see the first exercise in The Shaak Technique Book.