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In the Tangent tab whilst editing the curve, change the Constraint from Free to Natural [or vise versa - can't remember!] and then in Properties > Plane pick an alternative reference plane perpendicular to the current one.
Its always been in ISDX but doesn't come up very often, one of the techies explained it to me once - something to do with the maths of the surface - but can't remember the precise details,
Sean