Reading through the [math node docs][1], I came across a math function named "ping-pong".

The docs state that ping-pong does the following

>The output value is moved between 0.0 and the Scale based on the input value.

What in the world does that mean?  Can anyone tell me what the actual ping-pong function is?  What input values lead to an output of 0, and what input values lead to an output of "scale"? 

  [1]: https://docs.blender.org/manual/en/latest/compositing/types/converter/math.html