It converts a position vector. For example, consider frame A and a frame B, moving wrt A at 6 * A.x + 7 * A.y (initial pos at origin of A). Now a point (3, 8, 9) in B would be written as (3 + 6t, 8 + 7t, 9) in A. Thats what this function does. Since conversion of pos vectors from one frame to another differes from reexpression of vectors, I thought it would be good to have this function here.