I might have to delay my response until I get a chance to look at the information you refer to in the text. Hopefully I'll get to that tonight; otherwise I should be able to get to it tomorrow. I also owe you an explanation of torsion. In brief, torsion is the rate at which the plane in which you are moving changes with respect to position. The plane in which you are moving at an instant is defined by the cross product of the unit tangent and the unit normal vectors. The rate at which the direction of this vector changes with respect to position is equal to the rate at which it changes with respect to time, divided by the rate at which distance changes with respect to time--i.e., divided by speed (magnitude of velocity). It will take me a few minutes to put this into coherent typed-outnotation (and also to look up the notation used in the text, most specifically the notation for the unit vector parallel to T X N), but this is the idea.