Imagine you're off somewhere space. You're holding a ball in each hand.

1. You're motionless with reference to the Earth. You let go of the balls. They stay put.

2. You're moving away from the earth with a constant velocity. You let go of the balls. They continue to move at the same velocity as you do: This is indistinguishable from case 1.

3. You're accelerating away from the Earth (using rocket boots). You let go of the balls. They do not continue to accelerate with you.

4. You're motionless with respect to Earth, except that you're spinning. Think of the fastest you've ever spun. You'll be really dizzy when you stop. You let go of the balls. They fly off into space, because in order to stay spinning, they need acceleration toward the center of the spin plane. By letting them go, you stop that acceleration, and their trajectory changes (or really, their trajectory change stops).

Velocity is all relative, but acceleration is not. Spinning is acceleration.

---

Here's another example:

You occupy a cosmos consisting only of you and an apple. You accelerate away from the apple for a moment, then stop accelerating. You now have a constant velocity away from the apple, but it's meaningless to say which of you is actually moving. You each perceive the other as moving away at constant velocity.

Now imagine you start spinning. You feel like you're spinning, and visually, you perceive the apple orbiting you at a bizarrely fast rate. From the apple's perspective, you are spinning. These two perceptions are not symmetric.