You drag a point on the skew curve the way you drag a point on an input line.
When you change the points on the curve itself, the calculate toggle should be
set to off. If it is left on, the skew curve will annoyingly snap back into
its original position, leaving you mumbling inaudibly.
You should also be aware that editing the skew curve does not change the input
lines. While changing a point on an input line will change the skew curve,
there is no such reciprocal behavior when you edit the skew curve.
Dragging Points on The Skew Curve