Breath

"There is a rebound - a kind of ripple effect that happens naturally at the end of an exhalation. The problem is we can't DO it.

Instead, look for the precise moment which marks the end of an exhalation. Be completely relaxed and attentive to your spine.
See if you can notice the snake like micro-movements that are your spine's response to your breath.

The Strength of any rebound depends entirely on the strength of the initial impact - the harder something is thrown, the stronger the bounce...

...so the more strongly you concentrate on noticing the exact moment at the very end of an exhalation, the more you'll feel the effect."

From See How Yoga Feels by Su Sareen.