Taper and Twist a mesh in Vertex Shader

I’ve been trying to work out how to twist and taper a mesh in a vertex shader
based on ShaderX3, but I just can’t get my head around how to interpolate the factors, anybody tried this before or can explain it to me? section 1.3

Walked away to do the washing up, and stopping trying to work out their words, and got it before I started, stupid mind!
code= confusion
mind= of course!