This is not just true of slinkies - if you release a steel rod from the top, the top really starts falling before the bottom. If you hit a tennis ball with a racket, a wave has to travel from the strings to your hand before you feel it - and the ball is well on its way to the net by this time. If you hit a golf ball, the ball is well on its way to the hole before you can feel it. Rigid bodies aren't truly rigid and understanding this is essential to understanding the way the world works