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?

http://shiba.hpe.cn/jiaoyanzu/WULI/soft/Hlsl/ShaderX3.pdf 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!