How do you create separation say when making SL size arcs.
Trying to keep it simple. Upper/lower body separation is done by stabilizing the core by both tightening the abs/obliques and/or using a blocking pole plant. The legs (femurs) rotate in the hip sockets while the hip turns (twists) an intermediate amount in the direction the legs are turning.
There are three turn initiations that use upper/lower body separation. Rotation (upper body twists in the new turn direction), counter-rotation (upper/lower body twists in opposite directions) and anticipation (described in the first pp).
I hope this answers your question.