In the recently proposed angular momentum (AM) theory of collision-induced rotational transfer, the probability distribution of effective impact parameter (or torque arm) plays a central role as the means via which linear momentum is converted to AM. This was found empirically to be best represented by the function P(b(n)) = b(n)(-gamma). Here we show that the ubiquitous inverse power function is readily explained as resulting from the combined contributions to P(b(n)) that come from considering both angular and radial variability in a realistic intermolecular potential.