I don't understand the math but the "vector * (unity / vector)" does not do a divide and multiply in the normal way.
Hmm well taking time to look into it, <0, 0, 0, 1> / rotation returns inverse of rotation ... and multiplying vector by inverse of rotation is the same as doing vector / rotation using LSL built-in operations.
In any case it really didn't seem to do anything useful when i put it in the code yesterday to see the end results. (i checked both the original and simplified version of the code) That is, what it actually does is translating the movements of prim in sim coordinate system into translations relative to the prim in question -- i.e. moving the mouse so it makes the prim move to 'right' side of the sim actually moves the pointer to the right of board prim and so on. Problem being this can be rather counter-intuitive in plenty situations, and the grab function working in relation to camera orientation only makes it worse ^^;;