Question

How is it possible for a wave to travel across a slinky since the slinky is not moving back and forth?

Answer 1

Oscillations.

Step-by-step explanation:

When a slinky is not moving, it's said that has an equilibrium state. When a force deforms the slinky it begins to oscillate, because the elastic force inside is going to pull back the slinky, returning to the equilibrium point, resulting in a oscillated motion.

Answer 2

when you sling the slinky forward, the beginning of the slinky moves away from equilibrium position then back, the disturbance that moves down the slinky is called a pulse I think I'm not 100%