The hand can act like a bouncing ball
Release a rubber ball so it falls to the ground. It does not remain on the ground but bounces back up and then repeats the same cycle over again a number of times until it is finally still. If we treat our hand as if it were imitating a bouncing ball, then the impetus we need to repeat a note a second (third, fourth…) time originates from what we did to play the first note: no additional energy is required.
The use of this effect isn’t limited to repeating the same note, it can apply to any series of notes or chords of equal duration. Our physical intent can be limited to playing the first of the series, and simply allow the others to happen.
If the arm is in motion horizontally at the same time that the hand is bouncing, then the effect is like skipping stones at a lake. The stone makes contact with the water (keyboard) then leaves the keyboard to make contact again, further in the same direction. The only intentional motion required is the one initiating the process, the rest happens as if on its own.
The most useful way to apply this at the piano is to do it simultaneously in both arms, in a motion that begins near the extremes of the keyboard and works its way inwards towards the center of the keyboard.* This puts the two sides of the body into symmetric harmony with each other, one side aiding and advancing the progress of the other.
* in some cases there is enough momentum left for the hands actually to cross each other.