THERE WILL BE MORE HERE LATER!

I'm pretty proud of this diagram, tho, in how I hope it explains how displacement works.

The black curve is the terrain profile. The red curve is the displacement function. The mess at the bottom shows the final result as the pink line. Displacement moves the surface directly along the 'surface normal' vector - that's a line which is perpendicular to that exact point on the terrain.

The diagram has pink dots at each example point. Where the displacement function crosses its absolute sea level mark (a value of 0), the terrain is not displaced at all. That accounts for the first half of the dots. The other half of the dots are either 2 units 'above' or 2 units 'below' the orginal terrain surface and each of those is drawn with its associated 'surface normal' indicator.

Displacement works best when the underlying terrain is relatively smooth. There are a couple of ways that smoothness can come about. One is by setting the various Roughness dials in the terrain texture's leaves to low values (duh!) and another, less obvious way is to specify a non-zero Smallest Feature Size on the terrain texture's leaves.